Laurel Lake water quality, nutrients, and algae, summer 2011 1 H.A. Waterfield, W.N. Harman and M.F. Albright SAMPLING ACTIVITIES Site visits to Laurel Lake were made on 16 June and 25 July 2011; water samples were collected over the deepest part of the lake from the surface to the bottom, as well as from the inlet stream and outlet of the lake. These samples were analyzed for nutrients, important ions, algae groups, and algal abundance (See Table 1 below). Measurements were recorded on-site for temperature, dissolved oxygen, specific conductance, ph and Secchi disk transparency (water clarity). On 16 June, measurements were also made to assess the presence of optical brighteners, which are found in laundry detergents, but the results were inconclusive; the tannic materials originating from the upstream bog caused interference with the readings. FINDINGS & RECOMMENDATIONS Algal growth in Laurel Lake is currently limited by the availability of nitrogen; this is the root cause of the blue-green algae over-abundance. This is typical in lakes that have historically experienced a moderate level of phosphorus loading from the watershed (i.e. active grazing by livestock, septic system influx, sediment loading, etc.). In the present situation, phosphorus has been added to the lake system to the extent that nitrogen and phosphorus concentrations are no longer in balance with the requirements of the algae. This situation favors the growth of blue-green algae (cyanobacteria). The resulting noxious blooms are not part of a healthy ecosystem (they do not provide nutrition to plankton or other organisms). They can greatly reduce clarity, impose odor problems, and can pose human and animal health risks even with passive contact. To ultimately solve the problem, a great reduction in phosphorus loading would need to be achieved to the point where phosphorus is again the nutrient controlling algal growth. In the short term, if the Association wishes to stop the development of blue-green blooms, a proactive strategy must be developed. Going Forward Long-term Management Recommendations: To get at the root cause of the algae problem, nutrient loading (especially phosphorus) must be addressed by the Laurel Lake community, including those who live away from the shore within the watershed. Generally, all land uses within the watershed should be assessed; in the case of Laurel Lake, the dominant land use contributing phosphorus is residential development. The situation has progressed to the point that phosphorus is also released from the sediments during the summer without control of this internal source of phosphorus, reductions from the 1 This report was prepared for the Laurel Lake Association by the BFS as part of a contractual agreement.
watershed will be much less effective in solving the problem. In-lake phosphorus inactivation may be your best option to break the cycle of blue-green blooms (see the description below). Short-term Actions: If the desire of the LLA in the short term is to prevent blooms from occurring for safety and recreational concerns, a plan should be formulated in the winter or early spring (with consult of a professional). How will the situation be monitored? Once certain conditions are met, what is the course of action? 1. Develop of a strategy to monitor the lake (i.e. weekly monitoring of temperature and presence/absence of blue-green algae) 2. Establish criteria for initiating treatment, potentially one of the following (this list is not exhaustive): a. An algaecide and/or peroxide: The algaecide kills the algae and the peroxide degrades the toxins so they aren t dispersed in the water. b. Phosphorus inactivation: Buffered aluminum sulfate is used to remove algae and phosphorus from the water column and settle it on the lake bottom. This will also largely prevent the release of phosphorus from the sediments. c. Circulation: Blue-green algae thrive in calm conditions. Circulation can be used to deter their growth by providing constant (or daily) agitation in the surface waters. 3. Treat according to regulations, best practices, etc. before a bloom develops. You, or a consultant, will need to work with your regional DEC office to develop an approved strategy to manage some of the treatments listed above may not be permitted in your DEC Region. 4. If a significant bloom occurs, lake users must be informed and proper precautions should be taken to avoid exposure. The DEC and/or Department of Health in your county should also be informed they will likely follow up with testing or further guidance. RESULTS Nitrate: very low (below our detection level); this indicates that the algae are using all available nitrate. The amount of nitrate in the water column is limiting the algal growth. Blue-green algae are able to dominate because they can use nitrogen from the atmosphere; other algal groups are not able to do this, and so are outcompeted by the blue-greens. Phosphorus: Concentrations in Laurel Lake are moderately high. It is not likely the limiting factor for algal growth and that there is loading of phosphorus to the lake from sources in the watershed and internally from the substrate under anoxic conditions in near bottom waters. It is difficult to directly compare 2011 results with those from past testing, as we are unsure of the type/location of sample taken for past analyses. It is likely that this would be comparable to the 2011 surface sample. However, it seems that phosphorus concentrations have decreased since the 1983 sampling. On the 25 July sampling, near-bottom phosphorus levels were showing an increase. This situation in consistent with internal loading, where phosphorus associated with bottom sediments is released into the water column in the absence of oxygen (see below) and becomes available for use by algae.
Algae and Water Transparency: Secchi disk transparency was 2.8m (9.2 ft) at both site visits; chlorophyll a concentration, a measure of algal pigments used to estimate algal abundance, was also similar on both site visits: 26 µg/l in June, 25 µg/l in July. These values are in a range that indicates eutrophic conditions. In June the algal community was dominated by chrysophytes, a group of algae that do well in cooler water temperatures and higher nutrient conditions. This provides more evidence in support of the conclusion that the lake is continuing to show signs of eutrophication, as indicated by previous monitoring and recommendations. In July, cyanobacteria (blue-green algae) were equally as abundant as the chrysophyte group, though they were not blooming at this point. Observations reported by members of the LLA indicate that a bloom did occur following a period of hot dry weather; this type of weather, along with low nitrogen availability favors blue-green algae over others. Any effort to control blue-greens needs to be initiated at the early onset of a bloom. Additional sampling may provide insight into when that might occur (seasonal succession of algae types, nutrient sampling (nitrogen: phosphorus ratio, temperature, etc.). Temperature and Dissolved Oxygen profiles indicate that Laurel Lake experiences strong thermal stratification during the summer months, meaning that there is a warm surface layer floating on top of the cold deep water (this is normal). This has major implications for the amount of dissolved oxygen in the water column and in turn, for habitat available to fish and other aquatic organisms, and very importantly, internal nutrient cycling. The combination of organic loading from the bog and moderate algal production contribute to oxygen loss in deeper water. On both sampling dates the entire water column below 4 meters depth (13 ft) was essentially devoid of free oxygen (anoxic), meaning that these depths do not provide habitat for fish or invertebrates (fish food organisms). These conditions also lead to internal phosphorus loading (the release of phosphorus from the bottom sediments) as previously discussed. Chlorides: Chlorides can be used to indicate potential contamination from onsite wastewater treatment systems, road salting, and other activities on the landscape. Chloride levels in Laurel Lake are extremely low; changes in these concentrations over time would likely indicate that chlorides are being added to the lake system from a land-based source. Total Nitrogen: Results indicate that organic nitrogen is a major component of the total nitrogen present in the lake; sources of this include algae in the water and organic materials flowing in from the upstream bog. Nitrogen in this form is not readily available for use by algae or rooted aquatic plants, and so is not contributing meaningfully to the over production of algae; however, it is partially associated with tannins and other substances that contribute to the brownish color of the water. These compounds have the effect of reducing light penetration, a condition that also favors blue-green algae that compete well under lowlight conditions. Ammonia: Not present in high concentrations through most of the water column. Increased concentrations in the deeper water result from the low oxygen conditions. Ammonia can be toxic to aquatic life, but in this situation it is likely not something to be concerned with.
ANALYSIS METHODS & RESULTS TABLES Table 1. Methods used in the analysis of water samples collected from Laurel Lake. Parameter Minimum Detection Level Method Reference Total Phosphorus 4 µg/l Persulfate digestion followed by Liao and Marten single reagent ascorbic acid 2001 Cadmium reduction method Total Nitrogen 0.2 mg/l following peroxodisulfate Ebina et al. 1983 digestion Nitrate+nitrite-N 0.2 mg/l cadmium reduction method Pritzlaff 2003 Ammonia-N 0.2 mg/l Phenolate method Liao 2001 Alkalinity Titration to ph = 4.6 APHA 1989 Calcium EDTA titrimetric method APHA 1989 Chloride Mercuric nitrate titration APHA 1989 Table 2. Nutrient concentrations determined for water samples collected from Laurel Lake on 16 June and 25 July 2011. BD indicates that the concentration was below detectable levels. Depth ammonia (mg/l) nitrate+nitrite (mg/l) total nitrogen (mg/l) total phosphorus (ug/l) (m) 6/16/2011 7/25/2011 6/16/2011 7/25/2011 6/16/2011 7/25/2011 6/16/2011 7/25/2011 0 BD BD BD 0.22 0.35 12 21 3 BD BD BD 0.19 0.47 20 31 6 BD BD BD 0.25 0.35 29 31 9 BD BD BD 0.29 0.58 25 35 12 0.34 BD BD 0.71 1.12 34 60 Outlet BD 0.04 0.20 0.84 11 49 Inlet BD BD 0.29 0.35 40 127
Table 3. Alkalinity and ion concentrations determined for water samples collected from Laurel Lake on 16 June and 25 July 2011. Depth (m) alkalinity (mg CaCO3/L) calcium (mg/l) chlorides (mg/l) 6/16/2011 7/25/2011 6/16/2011 7/25/2011 6/16/2011 7/25/2011 0 7.2 7.2 3.6-3.5 4.2 3 7.2 6.2 4.0-4.5 4.2 6 7.2 6.2 4.4-4.0 4.5 9 9.2 11.3 4.0-4.0 4.5 12 10.3 12.3 4.0-4.0 4.5 Outlet - - 0.0 - - - Inlet 3.1 2.1 2.8-2.5 2.5 Table 4. Physical measurements made on 16 June and 25 July 2011 at Laurel Lake. Depth (m) temperature (C) dissolved oxygen (mg/l) ph sp. conductivity (µs/cm) 6/16/2011 7/25/2011 6/16/2011 7/25/2011 6/16/2011 7/25/2011 6/16/2011 7/25/2011 0 20.75 27.11 9.09 7.36 7.26 7.00 26 32 1 19.85 27.16 9.23 7.39 7.18 6.92 26 32 2 17.89 24.21 9.84 9.00 6.90 6.73 26 32 3 10.64 16.81 10.40 5.11 6.63 6.25 28 33 4 6.98 10.78 4.67 3.51 6.38 6.18 28 33 5 5.90 7.08 1.90 0.00 6.32 6.13 28 34 6 5.39 6.03 0.00 0.00 6.33 6.11 28 34 7 5.10 5.45 0.00 0.29 6.43 6.33 32 40 8 4.96 5.21 0.00 0.00 6.48 6.42 35 43 9 4.92 5.13 0.00 0.00 6.55 6.43 40 50 10 4.89 5.06 0.00 0.00 6.64 6.45 44 53 11 4.88 4.98 0.14 0.00 6.82 6.45 72 60 12 4.87 4.98 0.12 0.07 6.71 6.44 88 80
REFERENCES APHA, AWWA, WPCF. 1989. Standard methods for the examination of water and wastewater, 17 th ed. American Public Health Association. Washington, DC. Ebina, J., T. Tsutsi, and T. Shirai. 1983. Simultaneous determination of total nitrogen and total phosphorus in water using peroxodisulfate oxidation. Water Res. 17(12):1721-1726. Liao, N. and S. Marten. 2001. Determination of total phosphorous by flow injection analysis colorimetry (acid persulfate digestion method). QuikChem Method 10-115-01-1-F. Lachat Instruments. Loveland, Colorado. Liao, N. 2001. Determination of ammonia by flow injection analysis. QuikChem Method 10-107-06-1-J. Lachat Instruments, Loveland, CO. Pritzlaff, D. 2003. Determination of nitrate-nitrite in surface and wastewaters by flow injection analysis. QuikChem Method 10-115-01-1-F. Lachat Instruments. Loveland, Colorado.