Sterner, R.W.
Abstract:
Phytoplankton are a complex food resource. Early attention was given mainly to how differences in size affect grazing pressure. Like other plants, algae vary tremendously in their composition between and within species. Today, we see much effort going into understanding how algal chemical composition influences zooplankton.
Phosphorus is a key element in regulating algal growth in lakes. When algae are cultured under a range of growth rates with P as the factor limiting their growth, they show a wide range in the ratio of C:P within the cells, from < 100 to > 1000 (molar). Lake bioassays indicate that high C:P ratios are indicative of low "Relative growth rate". Such differences in C:P ratios have been reproduced in cultures, and then fed to live Daphnia to see how growth differs. In these experiments, which will only briefly be covered in this talk, the C:P ratio or the %P of the algae are strongly correlated with Daphnia growth, indicating either a true causal link or a strong indirect causality. Daphnia individuals reared on P-limited algae are small and have high lipid stores, indicating a discrepancy between lipid uptake and body mass growth. Daphnia are probably the most sensitive of the zooplankton to C:P in the food since they have the highest content of P within their bodies of all zooplankton so far measured.
A whole-lake manipulation of the food web in an oligotrophic lake in Ontario, Canada, will be described. This lake was formerly dominated at the top of the food web by four species of cyprinid minnows. In early 1993, myself and several colleagues stocked the lake with northern pike. We expected to see the response reported by others many times, namely reduction in minnows, increase in Daphnia, and reduction in algae. In contrast, we saw dramatic declines in minnows, but not a large increase in Daphnia. This lake, like many small lakes on the Precambrian Shield in Canada has seston of very high C:P ratio. We interpret the lack of food web response to pike as being due to a nutritional deficiency in the zooplankton.
We expect algal C:P ratios to be critical to zooplankton; thus we must understand the factors that cause C:P ratios to vary from lake to lake. To understand this question, I have examined the seston C:P ratio in a suite of > 100 Canadian lakes (data published by the NOLSS project). In these lakes, seston C:P correlates strongly with the ratio of the mean light in the epilimnion divided by total P. Thus, food quality correlates with the physical and chemical environment provided by the lake. Literature data linking light and nutrients to invertebrate production will be presented.
Tests of the influence of the light:nutrient ratio on Daphnia production were performed by myself and Dr. Jotaro Urabe. We grew algae under a range of light and nutrient levels and then recorded the growth rate of Daphnia on those foods. At all nutrient levels, zooplankton growth was maximal at intermediate light levels. At low light, primary and secondary production are positively correlated. At high light, they are negatively correlated due to food quality constraints.
Finally, a model of C and P metabolism will be presented. This model was written in an attempt to understand how food qualities tend to become identical at very low food, near the starvation threshold of the zooplankton. In this model, a carbon-specific respiration term was included, and production was held at a homeostatic ratio. This model shows how the boundary between C and P limitation in homeostatic herbivores is food quantity dependent. The C:P ratio necessary to shift the herbivore into P-limited body growth is higher and higher at lower and lower food quantity.
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