Aquatic Plant Mapping and Water Quality Monitoring at Congamond Lake 2009

Transcription

1 Aquatic Plant Mapping and Water Quality Monitoring at Congamond Lake 2009 Prepared for: Conservation Commission Southwick, MA Prepared by: Northeast Aquatic Research Mansfield, CT March 11, 2010

2 Table of Content List of Tables...2 Introduction...3 Study Scope...4 Aquatic Plant Survey...4 Methods...4 Aquatic Plant Community Structure...5 Aquatic Plant Species Results...9 Water Quality Temperature/Dissolved Oxygen/Water Clarity Nutrients Alkalinity/pH/Conductivity Plankton Conclusions References List of Tables Table 1 - Dates of visits and tasks conducted during this study...4 Table 2 Aquatic plants observed in Congamond Lake during 2009 survey Table 3 Percent occurrence of aquatic plants observed during this study Table 4 Water sampling depths used during this study Table 5 Thermocline depths (meters) for Congamond Lake during Table 6 - Secchi disk depths (meters) for Congamond Lake during Table 7 - Location (meters below the surface) of Anoxic Boundaries In Congamond Lake during Table 8 - Lake trophic categories and ranges of indicator parameters Table 9 - Total phosphorus (ppb) testing results for Congamond Lake in Table 10 Ammonia nitrogen (ppb) in Congamond Lake during Table 11 - Total nitrogen (ppb) in Congamond Lake during Table 12 Alkalinity of Congamond Lake water during Table 13 ph values in Congamond Lake waters during Table 14 Calcium concentrations in Congamond Lake during Table 15 Conductance (µmhos/cm) values in Congamond Lake during Table 16 Total Iron (mg/l) values in Congamond Lake during Table 17 - Phytoplankton Counts in Cells/ml For Congamond Lake... 33

3 Introduction Congamond Lake is a 472 acre lake located on the border of Suffield, Connecticut and Southwick, Massachusetts. The lake consists of three basins, called South, Middle, and North Ponds. South Pond has a surface area of 147 acres and maximum depth of 26 feet. Middle Pond, the largest of the three, has a surface area of 278 acres and a maximum depth of 37 feet. North Pond, the smallest and the deepest of the three, has a surface area of 47 acres and maximum depth of 40 feet. Each of the three ponds are isolated from each other by shallow water tunnels under Congamond Road separating South and Middle Ponds, and Point Grove Road separating Middle from North Pond Congamond Lake showing South Pond, foreground, Middle Pond, center, and North Pond at the top of the picture.

4 Study Scope Northeast Aquatic Research was contracted to conduct a survey of aquatic plants in each of the three ponds and collect water quality information during the summer of 2009 (see Table 1 for dates of the visits). The aquatic plant survey consisted of collecting species presence at GPS waypoints along the entire shoreline of the lake. Two sets of water quality sampling trips were made using sites located in the deepest water of each of the three basins. Water samples were collected form basic water quality parameters, specifically plant nutrients phosphorus and nitrogen. A profile of water temperature and dissolved oxygen was collected by measuring these values at each one meter depth increment. One sample was collected for plankton enumeration and one sample was collected for Chlorophyll-a determination. Two samples of sediments were collected from different areas of the lake for analysis of residual diquat. Table 1 - Dates of visits and tasks conducted during this study Task September 9, 2009 September 14, 2009 October 14,2009 Water Quality Sampling X X Aquatic Plant Survey X Sediment Samples X Aquatic Plant Survey Methods The aquatic plant investigation was conducted on September 14, The survey consisted of observing the littoral zone along the entire shoreline of the lake. The littoral zone is the shallow water area of the lake that supports rooted aquatic vegetation. Water depths were continuously monitored using an electronic depth sounder to identify the width of the littoral zone. The aquatic plants occurring within the boundaries of the littoral zone were identified and growth characteristics recorded. Formal observation points were made using GPS to be later downloaded to Google Earth for map creation. At each formal observation point, all aquatic plant species present at that location were identified. Species presence at each point was determined by viewing the community from the boat until all readily identifiable

5 species were recorded. Samples of plants were then collected and brought into the boat using a modified rake attached to a 16-foot extendable pole. Specimens of species that could not be identified in the field were retained for closer examination later. Typically, observation points were located equidistant between the shoreline and the outer edge of the littoral zone; however, when the littoral zone was wide additional points were made so that the plant community between the shore and the outer edge of the littoral zone was assessed. Characteristics of beds, such as density, growth to the surface and percent composition were recorded at each observation point. In this way, 234 observation waypoints were made during the survey, 48 in North Pond, 105 in Middle Pond, and 81 in South Pond. The littoral zone between waypoints was visually inspected to verify that species composition did not change. The locations of the observation points are shown in Map 1. Aquatic Plant Community Structure Aquatic plants are an important part of lake systems. They support a diverse community of organisms, mostly invertebrates that support the food chain in lakes. Aquatic plants also intercept runoff, store nutrients, and stabilize sediments. Aquatic plants create habitat for fisheries, providing for spawning, nursery areas for young fish, cover from predation, and ambush sites for predators. Aquatic plants in lakes occur in three distinct habitat forms, emergent, floating-leaved, and submersed. 1. Emergent plants are those rooted in shallow water, between 0.5 and 4 feet of water, but have a majority of stems and leaves out of the water. Generally, these species grow along natural wetland shorelines where the soils are saturated. Rarely do emergent plants grow in water past about 1 foot of depth. Species in this group include cattails, bulrush, pickerelweed, and phragmites. 2. Floating-leaved plants are the water lilies, water shield, and a few of the pondweeds. These plants produce primarily floating leaves with little or no underwater leaf development. Water lilies are generally restricted to shallow waters of less than about 6 feet. A subclass of floating-leaved plants is the tiny free-floating

6 plants, duckweed and watermeal. These tiny plants, less than a quarter of inch in size, usually grow near the shore in quite waters. Map 1 Location of waypoints used in aquatic plant distribution survey of Congamond Lake conducted during this study

7 3. Submersed plants are those that grow entirely underwater growing out to deeper waters of the lake. Submersed plants include Eurasian milfoil and tape grass. These

8 plants are rooted in the sediments reaching various heights into the water column. Sometimes submersed plants can reach the water surface to form floating leaves or short aerial flowers. When some submersed plants reach the water surface, the shoots spread out and continue to grow forming dense topped-out growths. Eurasian milfoil commonly does this. Two of the pondweeds can do this to a lesser extent, large-leaf pondweed and floating-leaved pondweed. However, most native plants develop shoots that remain underwater, unless they grow in shallow water. Aquatic plants require sufficient light to grow. The area of the lake where submersed plants can occur is called the photic zone. This part of the lake is defined as the area where sunlight reaches the bottom. The maximum depth of the photic zone can be estimated using the Secchi disk depth (Canfield et al., 1985). For Congamond Lake, the average Secchi disk depth from all basins is 6.5 feet, suggesting the maximum depth plants can grow will be about 6.5 feet. However, North Pond was clearer than Middle and South Ponds with an average Secchi depth of 10 feet. South Pond was more turbid than the other basins with an average Secchi depth of 4.5 feet. This indicates that generally, plants will be found out about 10 feet in North Pond, but only to about 5 feet in South Pond. It is important to note that these depths are based on late summer readings. The water clarity is probably better in May and June suggesting that the photic zone has not been accurately identified by data collected during this study. The chart in Figure 1 shows the number of species found at different water depths. The highest numbers of species found at any observation point was 9. Generally, 5 to 7 species were found at observation points where the water depths were between 1 and 6 feet. No more than 3 species were found when water was between 6 and 10 feet deep, although one observation point in North Pond had 5 species in water 9.5 feet deep. There were only 5 observation points were a single species, either tape grass or large-leaf pondweed, occurred in water depths of 10 to 13 feet. No plants were found in water deeper than 13 feet. Figure 1 Relationship between number of species of aquatic plants and water depth

9 Number of species Water depth (feet) Aquatic Plant Species Results The aquatic plant survey of Congamond Lake revealed 38 species of aquatic plants. Two invasive, non-native, submersed species were observed, Eurasian milfoil and minor naiad. Two invasive, non-native, shoreline species were observed, common reed-phramites, and purple loosestrife. Eurasian milfoil was found at only two locations in the lake, one plant was found in North Pond, and a few plants in a loose cluster were found in South Pond. The minor naiad, a new invader to New England, first records in MA and CT are 1974 (1980), and 1995, respectively (Les & Mehrhoff 1999). Minor naiad was found at only two observation points, one in South Pond and one in Middle Pond. The locations of these sightings are shown in Map 2. The presence of the two shoreline invasive plants, phragmites and purple loosestrife were not specifically surveyed for in Congamond Lake. Notes were kept when their presence was obvious not each time they occurred. Because these plants can spread rapidly spread along shoreline edges the location of the sightings is shown on Map 1. Phragmites has been in CT/MA for many years having replaced or in the process of replacing cattails in many wetlands. Purple loosestrife is a more recent invader that has very similar habitat preferences. Phragmites was found noted in place in Middle Pond, purple loosestrife was noted in three sites in South Pond. Map 2 Locations of invasive aquatic plants in Congamond Lake Sept. 14, 2009

10 The complete list of species encountered during this survey is given in Table 2. The table divides the species into three habitat types emergent, floating-leaved, and submersed. There were 7 emergent species, 6 floating-leaved species, and 25 submersed species for a total of 38 species of aquatic plants.

11 The emergent species include Eastern Bur-reed, pickerelweed and spikerush. These shoreline plants typically grow in shallow water of wetlands edges. Eastern bur-reed and pickerelweed are large robust plants growing to about a 2 feet tall, pickerelweed has large spike of conspicuous purple flowers. The floating-leaved plants found included both white and yellow water lilies and aquatic smartweed. Both water lilies form large floating leaves several inches across and each has large flowers. The yellow water lily flowers early in the season sometimes as early as May, while white water lily produces flowers later in the summer. Aquatic smartweed forms floating shoots and leaves that spread over the water surface. There were three species of the tiny non-rooted floating plants, duckweed, watermeal and great duckweed. These are the tiniest of the vascular plants, watermeal is about 1 mm in diameter, while duckweed and great duckweed are slightly larger, up to about 5 mm. These species are generally found together in quite water, usually sheltered coves and small windless ponds. There was a high diversity of submersed plants in Congamond Lake. Six of the species were found in shallow water of only a few feet deep, leafless milfoil, golden pert, waterwort, aquatic moss, arrowhead and floating-leaf pondweed. The first four are small inconspicuous plants that have a short stature and may only be a few inches tall. Floating-leaf pondweed has floating leaves about 2 inches long that can form a dense surface mat. The remaining submersed species occurred at all depths between the shore and the outer edge of the littoral zone. As shown in Figure 1 most species were found in water depths between 2 and 6 feet. Table 2 Aquatic plants observed in Congamond Lake during 2009 survey Common Name Species Name Habitat

12 Emergent Species Eastern Bur-reed Sparganium americanium Along the shore in water up to 1 foot deep Cattail Typha sp. Shoreline typically above the water line Purple Loosestrife Lythrum salicaria Shoreline - typically above the water line Pickerelweed Pontederia cordata Along the shore in water up to 1 foot deep Spikerush Eleocharis sp. Along the shore in water up to 1 foot deep Common Reed Phragmites Shoreline - typically above the water line Floating-Leaved Species Water Willow Decodon verticillatus Shoreline - in water up to 1 foot deep Yellow Water-lily Nuphar variegata Large floating leaves yellow flowers White Water-lily Nymphea odorata Large floating leaves white flowers Duckweed Lemna minor tiny floating leaves not rooted Great Duckweed Spirodela polyrhiza tiny floating leaves not rooted Water Meal Wolffia sp. very tiny floating leaves not rooted Smartweed Polygonum amphibium Submersed Species Floating leaves and shoots typically rooted on shore and spreads over water surface Coontail Ceratophyllum demersum any depth shallow Muskgrass Chara any depth - to very deep Waterwort Elatine sp. shallow water < 3 feet Slender Waterweed Elodea nuttallii any depth (in LZ) Aquatic Moss Fontinalis sp. shallow water < 3 feet Golden Pert Gradiola aurea shallow water < 3 foot Eurasian Milfoil Myriophyllum spicatum any depth (in LZ) Leafless Milfoil Myriophyllum tenellum shallow water < 3 feet Water Naiad Najas flexilis any depth (in LZ) Southern Water-naiad Najas guadalupensis any depth (in LZ) Minor Naiad Najas minor any depth (in LZ) Large-leaf Pondweed Potamogeton amplifolius any depth (in LZ) Thread-leaved Pondweed Potamogeton bicupulatus any depth (in LZ) Grassy Pondweed Potamogeton gramineus any depth (in LZ) Pondweed x cross Potamogeton hybrid any depth (in LZ) Illinois Pondweed Potamogeton illinoensis mostly deeper water (in LZ) Floating-leaf Pondweed Potamogeton natans shallow water < 3 feet Clasping-leaved Pondweed Potamogeton perfoliatus any depth (in LZ) Small Pondweed Potamogeton pusillus any depth (in LZ) Robbins Pondweed Potamogeton robbinsii any depth (in LZ) Sterile submersed Arrowhead Sagittaria cristata shallow water < 3 feet Arrowhead Sagittaria gaminea any depth (in LZ)

13 Sago Pondweed Stuckenia pectinata any depth (in LZ) Tape grass Vallisneria americana any depth (in LZ) Yellow Star-grass Zosterella dubia any depth (in LZ) Bold species are non-native and invasive The percent occurrence of each aquatic plant species observed during this study is given in Table 3. The table lists the frequency at which each species was found as a percentage of the total number of observation points (234). The values indicate how common each species was at the time of the survey. Because the entire shoreline was surveyed, these occurrence numbers are good estimates of the relative dominance of each species in Congamond Lake in September Only 5 species had dominance values over 10%, 14 species had occurrences between 2 and 10%, and 20 species were found at fewer than 2% of the points. The most dominant aquatic plant in Congamond Lake at the time of the survey was tape grass, appearing at half (50%) of the observation points. The second highest occurring species was white water lily at 28% of the sites, yellow water lily, usually associated with white water lily preferring the same habitat conditions, occurred at 12% of the sites. In almost all cases, the two species were encountered together. The third most widespread species was Robbins pondweed at 18%. Interestingly, mats of filamentous algae occurred at 13% of the sites. Filamentous algae was more common than most of the aquatic plants observed in Congamond Lake. This type of algae typically grows as dense mats that cover the bottom in shallow water where the sediments are predominantly muck. Table 3 Percent occurrence of aquatic plants observed during this study Species Name Common Name Percent Occurrence Vallisneria americana Tape grass 50 Nymphaea odorata White Water Lily 28 Potamogeton robbinsii Robbins Pondweed 18 Filamentous algae 13 Potamogeton amplifolius Large-leaf Pondweed 12 Nuphar variegata Yellow Water Lily 12 Chara Muskgrass 7 Potamogeton bicupulatus Thread-leaved Pondweed 7 Najas flexilis Water Naiad 6

14 Potamogeton pusillus Small Pondweed 6 Typha sp. Cattail 6 Elodea nuttallii Slender Waterweed 5 Pontederia cordata Pickerelweed 3 Sagittaria cristata Sterile submersed arrowhead 3 Elatine sp. Waterwort 2 Myriophyllum spicatum Eurasian Milfoil 2 Potamogeton perfoliatus Clasping-leaved Pondweed 2 Eleocharis sp. Spikerush 2 Ceratophyllum demersum Coontail 2 Lythrum salicaria Purple Loosestrife 2 Najas minor Minor Naiad 1 Lemna minor Duckweed 1 Spirodela polyrhiza Great duckweed 1 Polygonum amphibium Smartweed 1 Najas guadalupensis Southern Water-naiad 1 Zosterella dubia Yellow Star-grass 1 Myriophyllum tenellum Leafless milfoil 1 Wolffia sp. Water Meal 1 Decodon verticillatus Water Willow 1 Sagittaria gaminea Arrowhead 1 Potamogeton gramineus Grassy Pondweed 1 Potamogeton hybrid Pondweed x cross 1 Gratiola aurea Golden Pert < 1 Fontinalis sp. Aquatic Moss < 1 Sparganium americanum Bur-reed -emergent < 1 Potamogeton illinoensis Illinois Pondweed < 1 Stuckenia pectinata Sago Pondweed < 1 Potamogeton natans Floating-leaf Pondweed < 1 Phragmites australis Common Reed < 1 The survey found a couple of pondweeds that could not be identified. Specimens of these plants were submitted for review and were identified as hybrids. These plants are marked as Pondweed x cross in Table 3.

15 Tape grass (Vallisneria americana) was found at half of the observation points. The locations were tape grass was found in Congamond Lake are shown in Map 3. Generally, areas along the littoral zone of the lake where plants were found contained tape grass. Map 3 - Locations of Tape grass in Congamond Lake September 14, 2009

16 The location of two water lilies, White Water Lily (Nymphaea odorata), and Yellow Water Lily (Nuphar variegata) are shown in Map 4. The largest beds were in South Pond. Some small beds were found in Middle Pond, but these beds were usually of fewer plants. There were also small beds of water lilies along the west shore of North Pond. Map 4 - Locations of White and Yellow Water Lilies in Congamond Lake September 14, 2009

17 The two pondweeds, Robbins Pondweed (Potamogeton robbinsii) and Large-leaf Pondweed (Potamogeton amplifolius) were found sporadically around the lake (Map5). There were large beds of Robbins pondweed in South Pond, along sections of the west and south shores, generally in areas were water lilies were also present. There was also a large bed of Robbins pondweed in South Pond. Large-leaf pondweed was not found to form large beds anywhere in the lake. Map 5 - Locations of Large-leaf pondweed and Robbins Pondweed in Congamond Lake on September 14, 2009

18 Water Quality Three in-lake stations were established, one in of the three basins. Each station was located in the area of the deepest water of the basin. Map 6 shows the water depth contour map of Congamond Lake used as a guide in locating the deepest water of each basin. The location of each station is also shown; North Pond had a maximum depth of 38 feet, Middle Pond had a maximum depth of 36 feet and South Pond had a maximum depth 25 feet. The site of deepest water in North Pond was close to where the map indicated the maximum depth would be, but in Middle and South Ponds, the location of deepest water was found in to be different, indicating that map is inaccurate. At each lake station, the water clarity was measured with a Secchi disk, and the water temperature and dissolved oxygen of the water was measured at each one meter depth increment. Water samples were collected from three depths, top, middle, and bottom, depths given in Table 4, for analysis of total phosphorus, ammonium nitrogen, nitrate nitrogen, total kjeldahl nitrogen, alkalinity, conductivity, ph, calcium, and total iron. One water sample was collected from 1 meter depth during each visit from Middle Pond for determination of Chlorophyll-a. One vertical plankton tow was made during each visit to collect a sample of phytoplankton and zooplankton organisms in the water column. In addition, two sediment samples were collected for analysis of the herbicide diquat. One was collected from Middle Pond and one from South Pond. Sediment was taken from the top 5 cm within the 2009 treatment areas. Table 4 Water sampling depths used during this study South Pond Middle Pond North Pond Top 1 meter (3 feet) 1 meter (3 feet) 1 meter (3 feet) Middle 3 meters (10 feet) 6 meters (20 feet) 5 meters (16 feet) Bottom 7 meters (23 feet) 10 meters (33 feet) 11 meters (36 feet)

19 Map 6 Congamond Lake water depth contours in feet. Also shown are locations of water quality stations and sediment sampling sites Temperature/Dissolved Oxygen/Water Clarity Water Temperature A lake can be initially characterized by the seasonal patterns of water clarity and water temperature. These two factors interact to give the lake its thermal configuration, that is, the layers of upper warm water and deeper cooler water. This

20 layering, called stratification, is the fundamental aspect of a lake that governs many of the important processes that occur during the year. The pattern of layering is repeated each year, usually with a high degree of regularity. Each season, lakes progress through a series of steps beginning with ice-out in the spring, warming during the summer, and cooling in the fall. In the spring, after ice-out, the lake has uniformly cold water from top to bottom. As the sun increases in strength during April and May, the water at the surface warms due to the penetration of sun s rays into the water, with the depth of warming determined by water clarity. Warmed water floats over colder deep water that receives little sunlight. The warm upper layer is mixed by wind action and generally has sufficient dissolved oxygen because it is in equilibrium (oxygen from the air saturates the water evenly) with the atmosphere. The warm surface water layer is called the Epilimnion. The cold dark bottom water typically loses oxygen during the summer months. The cold-water layer is called the Hypolimnion. During stratification, those two layers are separated by a boundary called the Thermocline, also called the Metalimnion or middle layer. During the late summer and early fall the water in the lake begins to cool and eventually the whole lake becomes isothermal, or has the same temperature from top to bottom. Once the lake is isothermal, it continues cooling uniformly until winter when ice forms on the surface. The water temperature measurements collected during this study shows the lake developed strong stratification by the time of the first measurements on September 9, Figure 2 shows the mixing resistance values for each of the three basins of Congamond Lake on September 9, The mixing resistance is a unit-less number that describes the difference in water density between each one-meter water depth. Values over 30 indicate weak stratification, values over 60 indicate strong stratification. The maximum value of mixing resistance demarcates the location of the thermocline.

21 Figure 2 Mixing resistance values for the three basins of Congamond Lake on September 9, 2009 Mixing Resistance -unitless Water Depth -meters North Pond Middle Pond South Pond The location of the thermocline in each basin during this study is given in Table 5. Thermocline depths in Middle and North Ponds were similar in September, between about 7.5 and 8 meters. This means that waters shallower than this depth were epilimnetic waters of uniform water temperatures, and water below this depth were hypolimnetic waters isolated from the atmosphere. South Pond had a thermocline depth of 5.8 meters, shallower than the other two basins. In October, the thermocline had migrated downward by 2.5 meters in Middle Pond, and 1 meter in North Pond, while completely disappearing in South Pond. These values represent late summer conditions when thermocline structure has already begun to erode downward due to decreasing solar energy and cooler air temperatures. Mid-summer thermocline depths are probably going to be slightly deeper than the Secchi depths, about 4 to 5 meters. Table 5 Thermocline depths (meters) for Congamond Lake during 2009 Date South Pond Middle Pond North Pond none

22 Water Clarity Secchi Disk Secchi disk depth is a way of measuring water clarity. Clarity of lake water is one of the most fundamental and important aspects of lake condition. Clarity of water is the most valued aesthetic element of a lake; transparent water signifies health and cleanliness. The universal method of measuring water clarity is by the Secchi disk. Several types of disks are used today, but each variation traces back to an all white disk used in 1865 by Father Pietro Angelo Secchi, a Jesuit astronomer and science advisor to the Pope (Hutchinson, 1975). He is credited as the first to measure water clarity using a lowered disk, so the disk is named for him. The disk is lowered into the water until no longer visible, and then slowly raised until it is visible again; the average of the two depths is recorded as the Secchi disk depth. The depth measured by a Secchi disk is where approximately 10% of the surface light reaches. Below the Secchi disk depth, light penetration tails off exponentially becoming completely dark at about twice the Secchi disk depth or the depth where < 1% surface light reaches. Light transparency can be lessened by anything in the water that increases the turbidity. Very fine silt particles can stay suspended in the water column for many days, especially if there is continuing mixing. Commonly however, the decrease in Secchi disk depth is attributed to increased abundance of algae in the water. Algae growth is a result of higher levels of phosphorus in the water so Secchi disk depth is also related to the quantity of phosphorus in the water. These three factors: water clarity, algae abundance, and phosphorus concentration are the three most important factors used to describe lake condition. Water clarity readings taken in Congamond Lake during this study are given in Table 6. During this study, South Pond had the poorest clarity readings, mean of 1.3 meters, while North Pond had the best clarity readings, mean of 2.8 meters. The large difference in water clarity between the different basins indicates a source of nutrients and algae production occurs in South Pond. The poorest clarity reading in South Pond was 1 meter in October, while on the same date the clarity in North Pond was 3 meters.

23 Table 6 - Secchi disk depths (meters) for Congamond Lake during 2009 Date South Pond Middle Pond North Pond Average Dissolved Oxygen Dissolved oxygen, or oxygen, is a gas that dissolves in water from diffusion out of the atmosphere, and from photosynthesis by aquatic plants. The temperature of the water directly affects the maximum quantity of oxygen that can be dissolved in the water with colder water containing more than warmer water. During the early spring, after the ice has left the lake, dissolved oxygen from the atmosphere saturates the entire water column. Once the thermocline begins separating the lake into upper and lower layers, the oxygen content in the deepest waters dwindles because there is no re-supply from the now cut off atmosphere. This process continues during summer when the thermocline is strongest. The dissolved oxygen content in the water under the thermocline continues to decline until it becomes depleted. One dissolved oxygen was been depleted the water is referred to as anoxic. The process of dissolved oxygen loss starts at the sediment surface typically in late May or early June, but quickly rises into the water column during summer. The ascending anoxic boundary reaches a maximum depth sometime in mid to late summer based on the equilibrium between oxygen diffusing downward from the atmosphere and the dissolved oxygen consumption rate at the bottom. In the late summer and early fall the upper water cools causing the thermocline to descend bringing oxygen with it so that by mid-fall oxygen has mixed all the way to the bottom. The anoxic boundary was prominent in Congamond Lake during September but was located at different depths in each of the three basins (Table 7). In North Pond, the boundary was located at 7.8 meters, with a total depth of 11.5 meters there was 5 meters of anoxic water between 8 meters and the bottom. In Middle Pond, the anoxic boundary was located at 5.8 meters, with a total depth of 11 meters there was 6 meters of anoxic water between 6 meters and the bottom. In South Pond, the anoxic

24 boundary was located at 5 meters, with a total depth of 8 meters there was 3 meters of anoxic water between 5 meters and the bottom. In October, the lake was becoming mixed so the anoxic boundary was located at deeper depths in the water column. In Middle and North Ponds, the anoxic boundary was located at about 9 meters, with 2 to 3 meters of anoxic water remaining over the bottom on that date. In South Pond, the anoxic boundary had disappeared as oxygen rich water had mixed to the bottom. Table 7 - Location (meters below the surface) of Anoxic Boundaries In Congamond Lake during Date South Pond Middle Pond North Pond none Typically, the anoxic water at the bottom of a lake contains high amounts of dissolved constituents including, phosphorus, nitrogen, and iron because without oxygen these materials can remain dissolved in the water. In the presence of oxygen iron precipitates usually taking phosphorus with it, and nitrogen, generally as ammonia, is oxidized to nitrate or assimilated into organic forms. Because phosphorus can accumulate in anoxic water the juxtaposition of the anoxic boundary and the thermocline is critical. When the anoxic boundary is close to, or above the thermocline, phosphorus can diffuse upward into the epilimnion where it becomes available to phytoplankton. The chart in Figure 3 shows the location of both the thermocline and the anoxic boundary in each of the three basins on September 9, The anoxic boundary was above the thermocline in both the South and Middle Pond on that date. This condition is a precursor for internal loading of phosphorus in Congamond Lake.

25 Figure 3 Location of Anoxic Boundary and Thermocline on September 9, 2009 in Congamond Lake 0.0 Basin South Middle North Sept. 9, Depth (m) Anoxic Boundary Thermocline Nutrients Phosphorus Phosphorus is the nutrient that is in the shortest supply in fresh waters. Plants as the foundation of the food chain require a number of essential elements for growth, but phosphorus has the highest ratio of supply and demand. Almost three times the next highest on the list -nitrogen. Phosphorus is the nutrient that determines the overall health of lakes. As phosphorus increases plant growth is stimulated. Higher abundance of algae in the water column decreases the penetration of light, linking phosphorus to water clarity. Phosphorus originates principally from the weathering of rocks in the drainage basin. Watershed disturbance releases phosphorus because it is bound tightly with soils, whenever soil erosion occurs phosphorus will be transported with the silt. All impervious surfaces will be a source of phosphorus. Most domestic waste water systems are a source of phosphorus. Phosphorus is also part of rain and dry fall. Waterfowl can be source of phosphorus if their numbers are high enough. The CT DEP differentiates the trophic condition of lakes using phosphorus, nitrogen, Secchi disk depth, and chlorophyll a, see Table 8. The nutrients phosphorus and nitrogen are the growth factors and Secchi disk depth and chlorophyll-a are the response indicators, in that adding the nutrients (phosphorus primarily) results in decreases in Secchi as the chlorophyll-a increases. Phosphorus concentrations

26 observed in Congamond Lake during this study are given in Table 9. All values were above 10 ppb so the lake is clearly not oligotrophic. Values in South and Middle Ponds were always above 20 ppb with most values above 30 ppb. The bottom value at South Pond in Sept. was 252 ppb, and from Middle Pond in Oct. was 196 ppb indicating accumulating phosphorus on those dates. North Basin phosphorus was low at the surface between 10 and 20 ppb, and moderate at the bottom, between 30 and 40 ppb. Table 8 - Lake trophic categories and ranges of indicator parameters Category T.P. (ppb) T. Nitrogen (ppb) Secchi Depth (m) Chlorophyll a (ppb) Oligotrophic Oligo-mesotrophic Mesotrophic Meso-eutrophic Eutrophic Highly Eutrophic Source = CT DEP 1982 Table 9 - Total phosphorus (ppb) testing results for Congamond Lake in 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface Middle Bottom Average The phosphorus concentrations in Congamond Lake shows strong evidence for internal loading occurring in South Pond and possible evidence for internal loading in Middle Pond. North Pond appears to have low internal loading with good phosphorus concentrations in upper waters. South Pond may have significant enough internal loading rates combined with the shallower depths allowing for quicker mixing and loss of thermocline structure that phosphorus is exported to Middle Pond. This would be the case if epilimnion waters in the South and Middle basins were more or less in equilibrium. As phosphorus diffuses to the epilimnion in South Pond, the higher phosphorus levels there will migrate to Middle Pond. With only late summer data from one year this is conjecture at this point. With only two data points (Sept. and Oct.), it is impossible to tell if the lake is trending toward more of less phosphorus in the water. Lakes can show seemingly

27 large annual variation when the phosphorus concentration is this low. The relationship between phosphorus and water clarity in Connecticut lakes is shown in Figure 4. The Connecticut lake data from the 1970s are shown as small circles (data from Frink and Norvell, 1984). The relationship shown in Figure 4 illustrates the non-linear relationship between phosphorus and water clarity. As phosphorus concentration increases water clarity decreases, first rapidly and then slower until the phosphorus concentration reaches about 30 ppb, after which water clarity is not further decreased with higher phosphorus concentrations. This indicates that water clarity decreases, caused by phytoplankton in the water column, is very sensitive to changes in phosphorus when the concentration is low. < 20 ppb, but at higher phosphorus concentrations, > 30 ppb, the amount of algae in the water column is sufficiently great enough to cause growth limitation due to self-shading. Once a lake reaches this condition, phosphorus greater than 30 ppb, the production of phytoplankton and recycling of phosphorus are resistant to change because the phosphorus is internally held by either living or dead algae and the magnitude of dead algae sinking to the bottom is large enough to maintain anoxic conditions there ensuring further internal loading. This negative feedback loop perpetuates the eutrophic condition. Figure 4 Relationship between phosphorus and Secchi disk depth in CT Lakes, showing values for Congamond Lake basins during Oligotrophic Secchi Disk Depth (m) North Pond Meso-Oligotrophic Mesotrophic Meso Eutophic Eutrophic 2 Middle Pond South Pond Highly Eutrophic TP (ppb)

28 Nitrogen Nitrogen is the nutrient in second highest supply to demand for plants after phosphorus. There is more nitrogen available usually so it is not as needed as phosphorus is. However, it is still a necessary ingredient for plant growth. Nitrogen tends to be of poor supply in lake sediments because of transformation into nitrogen gas. Because of the loss of nitrogen from the sediments through the gases phase aquatic plants can be limited by nitrogen availability. Nitrogen occurs in three different forms in freshwater lakes. There are two ionic forms, nitrate, and ammonia, which will fluctuate depending on the concentration of dissolved oxygen present. An organic form is bound to other minerals and elements in complex compounds. These organic forms of nitrogen can be protein, amino acids, or partially decomposed plant material. Organic nitrogen is more refractory, requiring some decomposition that uses dissolved oxygen before being used as a plant nutrient. The ammonia concentration was very high in bottom water of both South and Middle Ponds in September (Table 10). In October, South Pond was mixed to the bottom causing ammonia concentrations to increase uniformly throughout the water column. Similar levels of ammonia were observed in Middle Pond in October but bottom levels continued to increase because that basin had not fully oxygenated to the bottom. North Pond showed low ammonia levels in the surface and middle depths, and moderate levels at the bottom. Table 10 Ammonia nitrogen (ppb) in Congamond Lake during 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface Middle Bottom 4, ,930 2, The levels of total nitrogen in Congamond Lake observed during this study are given in Table 11. Total nitrogen is the sum of organic nitrogen and nitrate/nitrite nitrogen. The levels of total nitrogen were very high in bottom waters and became very high throughout the water column in October because of the lake mixing.

29 Table 11 - Total nitrogen (ppb) in Congamond Lake during 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface 780 1, , Middle 690 1, Bottom 5, ,470 5,875 1,700 2,510 Alkalinity/pH/Conductivity Alkalinity - ph Lake water is in equilibrium with the carbon dioxide in the atmosphere. The gaseous carbon dioxide from the atmosphere dissolves into water and forms carbonic acid (H 2 CO 3 ), which immediately dissociates (splits into component parts), into bicarbonate and carbonate ions (HCO 3 and CO 3 ). The action of these two ions determines the alkalinity and ph of the water. Both these ions are negatively charged, binding readily to calcium and other positive ions in the water (others usually present at much lower levels than calcium are potassium and sodium). With more calcium in the water, more bicarbonate can accumulate as the compound calcium carbonate (CaCO 3 ). Alkalinity is the measure of how much calcium carbonate occurs in the water. Water with high levels of CaCO 3 alkalinity is considered hard water, usually over 50 mg/l CaCO 3. Most of Connecticut lakes have alkalinity values between 0 and 120 mg/l with a mean value of 20 mg CaCO 3. The ph of the water is a measure of the hydrogen ion concentration. When the gaseous carbon dioxide dissolves and dissociates into lake water a small amount of ionic hydrogen is released. The ionic hydrogen is written as H +. In chemistry, the accumulation of H + ions constitutes an increase in acidity. The opposite of the H + ion is the OH - ion. The accumulation of OH - ions increases the alkaline nature of the water. The two processes are opposed to each other, as acid and alkaline conditions cannot co-occur. The ph of the water is a measure of the amount of H + ions as the reciprocal log of its concentration, hence the little p indicating log value. As the H + increases, the ph goes down, acid values are between 0 and 7, and as OH - increases, the ph goes up, and alkaline values are between 7 and 14. When the two are equal, the ph is 7, indicating neutral conditions.

30 One further step is needed to complete our discussion of the ph relationship in fresh water. As already pointed out, carbon dioxide gas dissolves in water to form carbonic acid which is weakly acidic. Rain falling on the landscape contains some carbonic acid at a natural ph of 5.6. Rainwater that percolates through the soil dissolves calcium from the rocks and soils. The calcium goes into solution in the ground water and enters the lake as dissolved calcium ions with the ground water. Once in the lake, calcium binds with bicarbonate and carbonate making alkalinity. Air pollution, specifically nitric and sulfuric acids from coal plants and automobile exhaust, has caused the ph of the rain to decrease by adding H + ions to the atmosphere. Normally rain has a ph value of 5.6, when the ph is lower than this value the rain is referred to as acid rain. The nitric and sulfuric acids in the atmosphere cause the ph of the rain to be as low as ph 3, with ph of low 4 common. These acids are neutralized by the alkalinity of the water, which is defined as the quantity of CaCO 3 in the water. The increase of H + from the acidic rain is neutralized by the quantity of bicarbonate and carbonate ions (HCO 3 and CO 3 ) in the water. However, bicarbonate and carbonate neutralization of acid, or buffering, is dependant on the amount of calcium in the water, or the alkalinity. Alkalinity is low because the watershed is composed of rock material deficient in alkaline minerals; there is little calcium to wash into the lake. If the neutralizing capacity is low the ph of the lake declines eventually coming into equilibrium with the rain and stream water ph. Alkalinity and ph were measured in Congamond Lake during this study, alkalinity ranged from 22 to 104 mg CaCO 3 /L, ph ranged from 6.7 to 7.6 (see Tables 12 and 13). The CT DEP defined water bodies as acid threatened when the alkalinity is below 5 mg CaCO 3 /L. Congamond Lake is not acid threatened; instead, the ph data show the lake to be on the alkaline side of neutral. The data shows North Pond to have about half the alkalinity as South and Middle Ponds. Bottom water alkalinity was significantly elevated in South and Middle Ponds indicating generation of alkalinity components. Calcium concentration ranged between a low of 11 mg/l to a high of 104 mg/l (Table 14). Interestingly calcium decreased by about half between September and October lake wide, although alkalinity did not show a concomitant change. Because usually calcium is the dominant component of alkalinity, a 50% decrease in calcium should have caused a decrease in the alkalinity value. Because alkalinity did not change

31 other components such as chloride, sodium or potassium are needed in the water to balance the loss of calcium. Table 12 Alkalinity of Congamond Lake waters during 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface Middle Bottom Table 13 ph values in Congamond Lake waters during 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface Middle Bottom Table 14 Calcium concentrations in Congamond Lake during 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface Middle Bottom Conductivity - Conductance The conductance of the water is its ability of conduct an electrical current. The property of water to do so is attributable to the sum of all the ions in the water. The units mhos are the reciprocal of ohms, a standard unit of electrical resistance. The measurement of conductivity is typically dominated by calcium but the other common ionic minerals are included; potassium, magnesium, sodium, chloride, and sulfate. Conductivity values for Congamond Lake measured during this study varied between 115 and 266 µmhos/cm (Table 15). Conductivity was highest at the bottom where the water was anoxic and ionic materials accumulated. Again, the North Pond had the lowest levels, while South Pond had highest levels. Table 15 Conductance (µmhos/cm) values in Congamond Lake during 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface Middle Bottom

32 Total Iron Iron is natural occurring element in lake water. Generally, in the presence of dissolved oxygen iron is maintained at very low levels, < 0.1 mg/l. However, in bottom waters when dissolved oxygen has been depleted iron can accumulate as a dissolved ion (ferrous iron). In Congamond Lake, iron was very low in surface and middle depth waters but was at high levels in the bottom water of South Pond in September when the water was devoid of dissolved oxygen (Table 16). Middle Pond also showed iron accumulation but at a lesser degree than South Pond. Again, North Pond had lowest iron levels. Table 16 Total Iron (mg/l) values in Congamond Lake during 2009 South Pond Middle Pond North Pond 9-Sep 14-Oct 9-Sep 14-Oct 9-Sep 14-Oct Surface < < Middle Bottom Diquat in Sediment Two samples of lake sediments were collected from areas within the 2009 herbicide treatment areas (see Map 6 for collection locations). Each sample consisted of surfical sediments taken from the top couple of centimeters. Samples were shipped to SunLabs Inc. in Tampa FL for analysis. No diquat was detectable in either sample. Plankton Phytoplankton are simple single celled plants that float in the water column. Phyto means plant and plankton has its root in Greek for wandering. These microscopic plants occur either as single cells or as colonies that can take many shapes. Phytoplankton, or algae as they are commonly referred, are composed of organisms from many different taxonomic groups. Typically, Diatoms, and Chrysophytes (Goldenbrown algae), occupy cleaner waters and Green algae, and Bluegreen algae more eutrophic water. High numbers of cells in the water causes water to become green but it can also be brown or bluegreen. Bluegreen algae generally form the worst

33 blooms that are responsible for reduced water clarity during summer and fall. The loss of Secchi disk depth is usually directly attributable to an increase in the number of bluegreen algae cells in the water. Crustacean zooplankton are small aquatic crustaceans found in most lakes and ponds. They are mostly free swimming ranging in size from 0.4 to 2.5 mm. There are two groups of crustacean zooplankton, cladocerans and copepods. The most important cladoceran is Daphnia, also known as water fleas. These organisms are the major herbivores in lakes, grazing principally on phytoplankton in the open water. Large bodied cladocerians, are those over 1 mm in size, have very efficient filtration rates. Copepods are omnivorous feeding on a variety foods. Together, the crustacean zooplankton are important prey for juvenile and adult fish forming the foundation of the food chain. Phytoplankton The phytoplankton in the Congamond were collected from the water quality station in Middle Pond. Blue-green algae was very numerous in September, the total bluegreen cell count was more than 22,000 cells/ml (Table 17). The most abundant bluegreen alga taxon Lyngbya, although Anabaena, and in October Microcystis, also present in high numbers. Diatoms, mostly Tabellaria, were present in high numbers in September. Greens were present in but at lower numbers. Table 17 - Phytoplankton Counts in Cells/ml For Congamond Lake Taxon 9-Sep 14-Oct BLUE GREENS Anabaena 2,443 5,328 Gleocapsa 59 0 Microcystis 0 2,796 Oscillatoria Lyngbya 19,472 2,649 Total 22,181 10,785 GREENS Chlamydomonas Chlorella Coelastrum Dictyosphaerium Gloeocystis Mougeotia Scenedesmus

34 Total 1,104 1,222 DIATOMS Melosira 0 29 Synedra Tabellaria 2, Total 2, DINOFLAGELLATES Ceratium 0 15 Total Cells /ml 25,418 12,731 Zooplankton The densities of different groups of zooplankton collected in Congamond Lake during this study are shown in Figure 5. Cladocera zooplankton in Congamond were all small bodied organisms of less than 0.6 mm in size, were normally these animals are between 1 and 2 mm in size. The lack of large bodied Cladocera in the samples indicates intense grazing by fish populations, possibly either yellow perch or landlocked alewives. Other, mostly all small, zooplankton, organisms, were present at moderate to low numbers including copepods and calanoids. There were some rotifers but again these were also poorly represented. Figure 5 Zooplankton densities in Congamond Lake Congamond ZOOPLANKTON DENSITY 45.0 Animals / Liter Sep Oct Totals Rotifera Copeopod Nauplii Sm Copeopoda Lg Copeopoda Sm Cladocera Lg Cladocera