INTRODUCTION

Our research focused on categorizing and counting the various plankton species in Ravenswood Pond on the campus of Calvin College. It was our hope that our data could contribute to previous research so we may, in the future, be able to see larger trends and refine our ecological model of this pond. We identified and monitored plankton populations and observed fluctuations in density over time.

By sampling 4 sites in the pond we investigated the possibility that different species of plankton were present at various areas and depths in the pond. By correlating temperature changes and rainfall with plankton densities, we could speculate about the influences these factors had on plankton populations in the pond.

The plankton populations may give information concerning the inputs and outputs of Ravenswood Pond. The amount and type of available nutrients could influence the dynamics of plankton populations. In cooperation with other studies, including water chemistry in the pond and input channels, sediment nutrient analysis and nekton assessment, the data will be useful in evaluating the state of Ravenswood Pond. Given that information, questions of management can be addressed more effectively.

METHODS

We sampled 4 sites at Ravenswood Pond on five days during the fall (Figure 1). The sites were chosen to allow sampling near an input, an output, a poorly mixed area, and the deepest area of the pond. Pond depth samples ranged from 1.0- 2.0 meters. We collected our plankton samples with a Wisconsin-style plankton net. We took one sample from each site and placed the samples into jars. To facilitate the identification and counting of species we combined 1 mL of a sample with 1 mL of 10% formalin solution. One mL of this new solution was placed in a Sedgwick-Rafter counting cell. We then divided the counting cell into 4 horizontal fields. We counted 2 of these fields on 4x magnification under a compound microscope. Three replicates were made for each site and we averaged the plankton counts. We multiplied the average counts by 4 to account for the dilution and the number of fields counted per slide. These new numbers were the average numbers of each species per mL of the sample at each site. To calculate the number per mL in the pond, we placed the number per mL of the sample into Equation 1.

Equation 1. #/mL in the sample/ Volume of the Sample (mL) = #/mL in pond/ Initial Water Column Volume (mL)

Equation 2. Initial water column volume = pi x Radius of plankton net squared x Height of water column

These calculations allowed us to accurately compare plankton densities throughout the fall and between our sample sites.

This procedure was slightly modified during the last two sample analyses in order to better identify and categorize the smaller plankton species. When preparation of the cells was complete, 2 horizontal fields were counted under 4x magnification, as done previously. However, in order to assess and identify the very small phytoplankton, we divided the counting cell into 12 horizontal fields. Of these fields, two half-fields were counted using the 10x magnification. We added the two counts together and multiplied by 24 to account for the dilution and number of fields counted per slide. These new numbers were the average of each species per mL of a sample at each site. Equations 1 and 2 were used to convert them to number per mL in the pond.

RESULTS

Ravenswood Pond appeared cloudy and green-colored throughout. This appearance, characteristic of eutrophic ponds, changed throughout the fall. As phytoplankton densities declined, pond clarity increased and the color changed from deep green to lighter green.

Two trends are apparent in the data collected this fall: producer densities declined and consumer densities increased. Phytoplankton populations declined steadily throughout the months of October and early November (Figure 2). Anacyctis was the dominant species the entire sampling period. Anacystis and Polycyctis were very abundant in the beginning of the study (32230 and 21440 individuals per L, respectively), but their numbers declined significantly by November 13 (1835 and 1060 individuals per L, respectively). Aphanocapsa never reached densities over 2000 individuals per L, but obtained peak density around October 23 at 1960 individuals per L. In November, however, Aphanocapsa were fewer than 200 individuals per L. Ceratium, the dominant photosynthetic protist in the summer, also declined in numbers throughout the fall. The Ankistrodesmus population severely declined in November. Unfortunately no information is available about Ankistrodesmus numbers in October because this species was not recognized until November 6.

Zooplankton density oscillated in October but increased dramatically in November (Figure 3). Non-photosynthetic protozoa (ie., Ascomorpha) did not follow this pattern but instead declined steadily throughout the fall. Similar to Aphanocapsa, Crustacea and Rotatoria densities were high on October 23 (500 and 323 individuals per L, respectively). The Crustacea, represented by Bosmina, Cyclops and Nauplius larvae, declined slightly after October 23 (345 individuals per L), then increased dramatically during the week of November 11 to 670 individuals per L. Crustacea densities were largely determined by the Bosmina population. The Rotatoria, namely Keratella spp. and Brachionus, declined during the next two weeks to 226 individuals per L, but increased dramatically to 1100 individuals per L by November 13. Rotatoria numbers were dominated by a large population of Keratella.

Sampling sites A and C (Figure 1) were the densest regions of the plankton community (Figure 5). Site C always contained the largest amount of Anacystis and Polycystis, except on October 2 when site A had slightly more Polycystis (Tables 1-5). Aphanocapsa was also most abundant in sites A and C in October, but this trend was not consistent in November. Zooplankton densities were generally greatest in sites A and C.

DISCUSSION

Five main factors regulate the density of the phytoplankton community. The four abiotic components are temperature, light, nutrient and rainfall inputs and the major biotic regulator is grazing. Death and birth rates are the governing principles behind population dynamics. When death rates exceed birth rates the population will diminish, and vice versa.

Light availability and temperature greatly limit phytoplankton reproduction rates. Photosynthetic microorganisms depend on the sun to provide energy for food production. During the fall light periods get shorter and light intensity declines. Phytoplankton reproduction is impeded by these changes and their populations diminish. The unique lake effect in western Michigan harbors sporadic temperature changes, but the overall trend is a declining average daily temperature (Figure 4). It is hard for phytoplankton to live in cold water; therefore, their populations decline throughout the fall. Nutrients, particularly phosphorus, also limit aquatic productivity. Storm water runoff is a large contributor of inorganic ions to Ravenswood Pond, but we have found no significant correlation between storm water inputs and algae populations.

Rainfall is another factor that could influence plankton density. Heavy rains raise the water level and dilute the pond. When this occurs, Ravenswood Pond overflows into Presidents Pond and many organisms are flushed away. Every sampling date (except November 6) was preceded by a substantial precipitation event (Figure 4). The pond level tends to return to normal levels 3 days after a rain but our samples may reflect a component of dilution. The decline in phytoplankton density is predominantly the result of low light, cold temperatures and grazing (discussed below); but dilution and flushing may have played a role as well.

Grazing is the biotic factor that limits phytoplankton populations. The primary consumers in a pond ecosystem are the zooplankton and planktivorous fish. Although most of the biomass in the zooplankton diet consists of nanoplankton (species we were unable to count, given the type of collecting net), grazing of phytoplankton still occurs. In general, grazing by zooplankton will reduce phytoplankton numbers, but when this is coupled with the abiotic factors discussed above, a dramatic shift from high to low densities of phytoplankton will ensue. The best evidence for this phenomenon is seen in the November samples (Figures 2 and 3). Ankistrodesmus and other previously unrecognized species suffered a tremendous mortality rate between November 6 and 13, but the Crustacea and Rotatoria populations increased dramatically. Zooplankton may also have exhibited larger numbers because of the irregularly benign November temperatures. But, our data supports the strong possibility that grazing increased zooplankton birth rates and increased phytoplankton death rates.

The plankton community seems to be concentrated on the east and north sides of the pond. If this observation is correct, it may have resulted from the net movement of wind blowing north and east. Buoyant phytoplankton can be easily moved around, but zooplankton are mobile and are not as susceptible to wind. This phenomenon seems to be reflected in the data: phytoplankton densities are consistently greater in the east and north (sites A and C, respectively), but zooplankton are much less consistent. Water flow and drainage into Presidents Pond may play a role in shaping this heterogeneous community, but wind is likely the dominant factor.

WORKS CITED

Pennak, Robert W. Fresh-water Invertebrates of the United States. John Wiley & Sons, Inc., 1978.

Needham, James G. & Paul R. Needham. A guide to the study of Fresh-water Biology. Holder Day, Inc. San Francisco. 1962.

FIGURES- click on any figure to view in full size

Figure 1	Figure 2
Figure 3	Figure 4
Figure 5

TABLES

Table 1:Mean number of organisms per mL of water in Ravenswood Pond on October 2, 1998.

Table 2 Mean number of organisms per mL of water in Ravenswood Pond on October 9, 1998.

Table 3 Mean number of organisms per mL of water in Ravenswood Pond on October 23, 1998.

Table 4 Mean number of organisms per mL of water in Ravenswood Pond on November 6, 1998.

Table 5 Mean number of organisms per mL of water in Ravenswood Pond