Plankton and Water Quality

Results

Nitrogen and phosphorus analyses for 2002 are not completed at this time, however preliminary data indicates that the concentrations of these nutrients are similar to those observed during previous summers.

No clear trend in total phosphorus (TP) concentration over the last 3 years is apparent, although the 2001 concentrations are somewhat lower than 2000. This may be the result of differences in weather patterns, especially precipitation amounts, between the two years. An overall mean concentration of 10 ug/L TP (standard error 0.017) was observed over the three year study period in Burlington Bay.

Total nitrogen concentrations (TN) were similar in 2000 and 2001, while 1999 concentrations differed markedly. This discrepancy may be due to preservation methods. Samples in 1999 were frozen until analysis, while later samples were preserved with acid. The 2000 and 2001 data show a pattern of decreasing TN concentrations through mid-summer, after which concentrations remain stable until mid-fall. Average values from April through July are 0.42 mg/L. Average values from August through October are 0.33 mg/L.

The molar ratios of TN to TP were calculated for Burlington Bay using all available data from 1999 through 2001. The ratio of these nutrients influences the composition of the phytoplankton community because nutrient requirements vary greatly among the different algal groups. Typical ratios in lakes where plankton growth is not nutrient limited are approximately 60 (Wetzel 2001). Ratios lower than this value suggest nitrogen limitation, a situation that can favor the blue-green algae, a group that can use atmospheric nitrogen to meet its nutrient needs during growth. Smith (1983) found that blue-green algae were rarely observed at ratios above 64 by molar mass.

The nutrient ratios in Burlington Bay are generally high, suggesting that nitrogen limitation is unlikely. An exception occurred in late summer 1999, when N:P ratios below 50 were observed. In September 1999, blue-green algae were abundant and two dogs died after ingesting lake water containing large amounts of toxic blue-green algae. N:P ratios were higher in 2001 than in 2000. This may reflect differences in weather patterns; 2000 was a wet summer while 2001 was extremely dry.

Zooplankton

To date, all of the zooplankton for Inner and Outer Harbor has been analyzed, providing a basis of comparison of the two sites over four years.  In 1999, the sites were sampled six times from mid-August through the end of October.  In 2000, 2001, and 2002, the sites were sampled 11 times each year starting in late April through mid October.

Total zooplankton densities were low in 1999 with an Inner Harbor peak of 13 individuals per L in late August and an Outer Harbor peak of 25 individuals per L in late September (Figure 6).  Cladocerans and copepods were equally represented over these dates (Figure 7).

In 2000, peak abundances of 30 - 40 individuals per L were seen in mid June, while minimum abundances of 1 - 1.6 individuals per L were seen with the first sampling of the season in April.  The composition fluctuated throughout the season with a high density of copepods in early spring, changing to a higher cladoceran density in July and August, then changing back to a more equal representation in the fall (Figure 7).

In 2001, an Inner Harbor peak abundance of 51 individuals per L was seen in early June, directly following the minimum abundance of 0.5 individuals per L in mid May.  The Outer Harbor had a minimum abundance of 1.3 individuals per L in mid May as well, though the peak abundance of 82 individuals per L was recorded in early July.  The zooplankton composition followed similar changes as seen in 2000.

April 2002 had the minimum zooplankton abundance of 0.7 and 1.3 individuals per L for Inner and Outer Harbor, respectively.  Inner Harbor’s peak abundance of 88 individuals per L occurred in mid July, while Outer Harbor’s peak abundance of 55 individuals per L occurred in mid August.  Zooplankton composition followed similar patterns as seen in 2000 and 2001, with a peak cladoceran density in mid summer.

Using late August as a reference time, two-way analysis of variance (ANOVA) was used to test for significant differences in both density and percent copepods between years and sites.  No significant differences were detected between years or between sites, probably because of the large amount of variation in the data.  Further analyses are underway.

Phytoplankton

Analysis of samples collected during 2001 and 2002 is continuing.  To begin to explore the data we have completed to date, we compared the Inner Harbors sites in 2001 to the Outer Harbor sites in 2001.  Because stormwater enters the lake near shore and the breakwater separates the Inner Harbor from the rest of Burlington Bay, we might expect to see differences in the plankton living in the Inner Harbor compared to those at other sites.  Densities of zebra mussels are also greatest closer to shore, and these filter feeders might also be changing the structure of the plankton community.

A first examination of phytoplankton densities showed similar patterns at both the Inner and Outer Harbor sites in spring and early summer, but some differences later in the summer (Figure 5).  From late April through early July, total cell densities ranged between 0.25 and 2.3 million cells per L.  Cell densities in the Inner Harbor doubled to roughly 4 million cells per L in late July and August.  A large, approximately 6-fold, increase was noted in the Outer Harbor by August 27 while the cell densities in the Inner Harbor remained consistent from late July to the end of August.

Diatoms dominated the phytoplankton at both locations in April.  Diatoms continued to represent 30 – 40% of the algae enumerated in the Outer Harbor samples through early July.  Cryptophyte flagellates were the dominant algae in late July in both the Inner and Outer Harbors.  Cyanobacteria, or blue-green algae, were the most abundant algae in August, representing 60 – 80 % of the algae enumerated at both stations.

Rotifers

Analyses of the rotifer samples continue, but some preliminary analyses have been completed.  In the fall of 1999, total rotifer densities were low, generally less than 15 individuals per L, except in mid September, when densities were 20 – 65 individuals per L and in mid August at the Lakeside site, where the density was about 45 individuals per L (Figure 8).  Preliminary data from fall 2000 indicate a similar pattern may emerge (Figure 9).  Species composition was similar at all sites in 1999, with fairly even abundances in mid August.  In late August through September, the species Keratella dominated the assemblage (Figure 10).  Preliminary data from fall 2000 show a greater relative abundance of the species Polyarthra (Figure 11).

Wetzel, R.G. 2001. Limnology: lake and river ecosystems. Academic Press. San Diego, CA. 1006 pp.