Otsego Lake limnological monitoring, 2011

Transcription

1 Otsego Lake limnological monitoring, 2011 Holly A. Waterfield and Matthew F. Albright INTRODUCTION Otsego Lake is a glacially formed, dimictic lake (max depth 51m) supporting a cold water fishery. The Lake is generally classified as being chemically mesotrophic, although flora and fauna characteristically associated with oligotrophic lakes are present (Iannuzzi, 1992). This study is the continuation of a year-round monitoring protocol that began in The data collected in this report run for the calendar year and are comparable with contributions by Homburger and Buttigieg (1992), Groff et al. (1993), Harman (1994; 1995), Austin et al. (1996), Albright (1997; 1998; 1999; 2000; 2001; 2002; 2003; 2004; 2005; 2006; 2007; 2008), Albright and Waterfield (2009), and Waterfield and Albright (2010; 2011). Concurrent additional work related to Otsego Lake included estimates of fluvial nutrient inputs (Zaengle 2012), and descriptions of the zooplankton community (Albright and Zaengle 2012), chlorophyll a (Levenstein 2012), and nekton communities (German 2012; Bowers 2012; Waterfield and Cornwell 2012). MATERIALS AND METHODS Physiochemical data and water samples were collected near the deepest part of the lake (TR4-C) (Figure 1), which is considered representative of whole-lake conditions, as past studies have shown the Lake to be spatially homogenous with respect to the factors under study (Iannuzzi 1991). Data and sample collection occurred bi-weekly during open water conditions, May through December. Sampling was conducted on 17 February through the ice. Samples were not collected in March or April due to marginal ice conditions. Physical measurements were recorded at 2-m intervals between 0 and 20 m and 40 m to the bottom; 5-meter intervals were used between 20 and 40 m. Measurements of ph, temperature, dissolved oxygen and conductivity were recorded with the use of a YSI 650 MDS with a 6-Series multiparameter sonde which had been calibrated according to the manufacturer s instructions prior to use (YSI Inc. 2009). This was the first year that an optical dissolved oxygen probe was used (as opposed to a Clark cell type). Samples were collected for chemical analyses at 4-m intervals between 0 and 20 m and 40m and 48m; 10-m intervals were used between 20 and 40 m. A summary of methodologies employed for sample preservation and chemical analyses is given in Table 1.

2 TR4-C Figure 1. Bathymetric map of Otsego Lake showing sampling site (TR4-C).

3 Table 1. Summary of laboratory methodologies. Parameter Preservation Method Reference Total Phosphorus H 2 SO 4 to ph < 2 Persulfate digestion followed by single reagent ascorbic acid Liao and Marten 2001 Total Nitrogen H 2 SO 4 to ph < 2 Cadmium reduction method following peroxodisulfate digestion Pritzlaff 2003; Ebina et al Nitrate+nitrite-N H 2 SO 4 to ph < 2 Cadmium reduction method Pritzlaff 2003 Ammonia-N H 2 SO 4 to ph < 2 Phenolate method Liao 2001 Calcium Store at 4 o C EDTA trimetric method EPA 1983 Chloride Store at 4 o C Mercuric nitrate titration APHA 1989 Alkalinity Store at 4 o C Titration to ph= 4.6 APHA 1989 RESULTS AND DISCUSSION Temperature Figures 2a and 2b depict temperatures measured in profile (0 to 48m) at site TR4-C from 6 January through 26 July and 8 August through 22 December 2011, respectively. Surface temperature ranged from 0.46 o C below the ice on 17 February to 25.4 o C at the surface on 26 July. Temperatures at 48m reached the annual minimum of 2.93 o C on 6 January. Ice went off the lake on 12 April; spring turnover occurred between then and 3 May. Thermal stratification was evident by 18 May. Surface temperatures began to decrease after the profile collected 26 July and the thermocline occurred at greater depth until fall turnover, which occurred in late December (Figure 2b). Dissolved Oxygen Isopleths of oxygen concentration based on the profiles for the calendar year are presented in Figure 3. On 5 April, prior to the onset of thermal stratification (May), dissolved oxygen was between mg/l (at 48m) and mg/l (at the surface). The minimum observed DO concentration was 4.59 mg/l recorded on 22 November at 48m. This is the highest minimum, late season, bottom concentration recorded since 1988 (Iannuzzi 1991). This compares to a minima bottom concentration in 2010 of 3.25 mg/l on 26 October. In most years between 1995 and 2009, the bottom minimum was near or below 1.0 mg/l. The areal hypolimnetic oxygen depletion rate (AHOD), calculated at mg/cm 2 /day, remains well below the historical average (Table 2).

4 2a Temperature ( o C) /6/2011 2/17/2011 5/3/2011 Depth (meters) /18/2011 6/1/2011 6/15/2011 6/28/2011 7/13/2011 7/26/2011 2b. Depth (meters) Temperature ( o C) /8/2011 8/24/2011 9/9/2011 9/27/ /12/ /2/ /22/ /5/ /22/2011 Figure 2. Otsego Lake temperature profiles ( o C) observed at TR4-C between 6 January and 26 July (2a) and 8 August and 22 December (2b) Table 2

5 Table 2. Areal hypolimnetic oxygen deficits (AHOD) for Otsego Lake, computed over summer stratification in 1969, 1972 (Sohacki, unpubl.), 1988 (Iannuzzi, 1991), and Time Interval AHOD (mg/cm 2 /day) 05/16/69-09/27/ /30/72-10/14/ /12/88-10/06/ /18/92-09/29/ /10/93-09/27/ /17/94-09/20/ /19/95-10/10/ /14/96-09/17/ /08/97-09/25/ /15/98-09/17/ /20/99-09/27/ /11/00-09/14/ /17/01-09/13/ /15/02-09/26/ /16/03-09/18/ /20/04-09/24/ /27/05-10/05/ /4/06-09/26/ /18/07-9/27/ /8/08-10/7/ /27/09-10/19/ /26/10-10/7/ /19/11 10/12/ Calcium Calcium concentrations followed a typical seasonal pattern of fluctuation similar to that of alkalinity. Mean annual concentration at TR4-C was 50.0 mg/l, ranging from 44.1 mg/l at 4m on 9 September to 52.9 mg/l at the bottom on 22 November. Chlorides Mean chloride concentrations in Otsego Lake from 1925 to 2011 are shown in Figure 4. Between 1994 and 2005 mean concentration increased steadily at of rate of 0.5 to 1.0 mg/l per year (Figure 4). Since then, mean annual concentrations have been variable and have actually trended slightly downwards. The mean lake wide concentration in 2011 was 14.4 mg/l; in 2010 it was 15.5 mg/l. Chlorides in Otsego Lake have generally been attributed to road salting practices, with the greatest influx of the ion during spring snowmelt events or early-winter snow storms.

6 Chloride (mg/l) Year Figure 4. Mean chloride concentrations at TR4-C, Points later than 1990 represent yearly averages (modified from Peters 1987). Nutrients Total phosphorus averaged 8.5 µg/l in 2011 (when not including tests that were below method detection), ranging from below detection (< 4 µg/l) on multiple dates to 40 µg/l at 12m on 13 July. Concentrations tended to be fairly homogeneous from surface to bottom. No phosphorus release from the sediments was observed prior to fall turnover, as dissolved oxygen was present at concentrations sufficient to maintain iron-phosphorus bonds in sediment materials. Nitrite+nitrate-N averaged 0.46 mg/l; ammonia-n was not measured, as it is generally below detectable levels (<0.02 mg/l) unless dissolved oxygen is depleted in the bottom of the hypolimnion. Total nitrogen analyses, yielding a mean of 0.62 mg/l, indicate an average organic nitrogen concentration of about 0.16 mg/l over the year. This situation was nearly identical to that observed in 2010 (Waterfield and Albright 2011). Secchi disk transparency and chlorophyll a Chlorophyll a concentrations were determined for samples collected on 10 dates from May through September Average 0-20m composite chlorophyll a concentration was 1.5 µg/l (range= 1.0 to 2.4 µg/l). This is compared to a mean concentration of 1.9 µg/l in 2010, and it is the lowest average recorded value since at least A more detailed description of the temporal and spatial distribution of chlorophyll a is provided by Levenstein (2012). Secchi disk transparencies ranged from 3.0m on 3 May to a season-maximum of 10.1m on 13 July (Figure 5). The temporal variation of transparency differed from that observed in 2010 (Figure 6). Mean summer Secchi transparencies for all years available ( ) are given in Figure 7. The marked increase in transparency noted in 2009 continues to date, and is likely related to the filtration capacity of the growing zebra mussel population, as similar changes in water clarity and chlorophyll a have been documented concurrent with the

7 establishment and growth of zebra mussel populations elsewhere (e.g. Leach 1993, Waterfield et al. 2011). Also, over summers of 2010 and 2011, Otsego Lake s zooplankton community comprised a higher abundance of Daphnia spp., which had a mean length substantially greater than any year since 1990 (Albright and Leonardo 2011). It is not known if this is resultant of the establishment of zebra mussels or more a function of declining alewife (Waterfield and Cornwell 2012) 2011 Otsego Lake Secchi Transparency 0 3-May 18-May 1-Jun 15-Jun 28-Jun 13-Jul 26-Jul 8-Aug 24-Aug 9-Sep 27-Sep 2 Depth (meters) Figure 5. May through September Secchi transparencies at TR4C, Otsego Lake, Otsego Lake Secchi Transparency 0 18-May 26-May 4-Jun 15-Jun 1-Jul 15-Jul 3-Aug 12-Aug 26-Aug 9-Sep 24-Sep 2 Depth (meters) Figure 6. May through September Secchi transparencies at TR4C, Otsego Lake, 2010.

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