Effects of nutrients and planktivorous fish on the phytoplankton of shallow and deep aquatic systems

Ecology, July, 1996 by M. Proulx, F.R. Pick, A. Mazumder, P.B. Hamilton, D.R.S. Lean

INTRODUCTION

Nutrient levels and circulation patterns have long been identified as factors influencing phytoplankton community structure in aquatic systems (e.g., Pearsall 1932, Reynolds 1984). In lakes, nutrient levels influence total phytoplankton biomass (e.g., Sakamoto 1966, Dillon and Riglet 1974), taxonomic composition (e.g., Pick and Lean 1987, Sandgren 1988, Smith 1990) and size distribution of the community (e.g., Watson arid Kalff 1981, Watson et al. 1992).

Physical features such as lake morphometry, circulation patterns, and the presence or absence of thermal stratification may contribute independently to within- and between-lake differences in plankton community structure. Lakes with thermal stratification are fundamentally different from those without thermal stratification. Shallow lakes without thermal stratification tend to have higher phytoplankton biomass than deep lakes with similar levels of nutrients (Pridmore et al. 1985). Furthermore, circulation patterns are frequently responsible for strong seasonal and within-lake differences in plankton communities (e.g., Margalef 1970, Munawar and Munawar 1975, Straskraba 1980).

Phytoplankton levels are, at times, dependent on predation. Large-bodied Cladocera, in particular, have high grazing rates and can "clear" the water column of algae when their abundance is high (Lampert et al. 1986). Other predators such as larval insects and planktivorous fish may also influence phytoplankton biomass and composition either by changing zooplankton community structure or reducing total zooplankton biomass (Hrbacek et al. 1961, Brooks and Dodson 1965, Lynch and Shapiro 1981, Carpenter et al. 1985, McQueen et al. 1986, Vanni and Findlay 1990). Planktivorous fish tend to reduce the number of large-bodied zooplankton, particularly Daphnia spp., thereby releasing phytoplankton from grazing pressure and increasing phytoplankton biomass. However, not all fish manipulation experiments have demonstrated such an effect (DeMelo et al. 1992). Furthermore, controversy exists over the pathways by which this "trophic cascade" (sensu Paine 1980, Carpenter et al. 1985) can influence phytoplankton communities since fish or zooplankton manipulations may be accompanied by changes in nutrient availability (Threlkeld 1988, Vanni and Findlay 1990, McQueen et al. 1992) and thermal structure (Mazumder et al. 1990b). Several authors have suggested that the magnitude of the effects of grazers on phytoplankton is dependent on nutrient availability and may vary positively (Mazumder et al. 1988, Sarnelle 1992, Mazumder 1994) or negatively (McQueen et al. 1986) across a gradient of increasing enrichment.

In temperate lakes, the seasonal thermal stratification of the water column influences epilimnetic phytoplankton composition by changing the relative sedimentation rates of individual taxa (e.g., Reynolds 1984). In stratified waters, large cells, particularly diatoms, tend to settle out more rapidly than in a mixed or nonstratified water column and the size distribution of phytoplankton shifts toward small or motile cells. Generally small cells with high growth rates or those with strong buoyancy control will dominate. Both thermal stratification and the ratio of nitrogen to phosphorus have been used to predict the distribution and occurrence of specific phytoplankton taxa (Harris 1986, Jensen et al. 1994). Stratification may also provide a predation refuge for zooplankton, thus potentially allowing zooplankton control of phytoplankton populations (Gliwicz 1986, Wright and Shapiro 1990). Furthermore, mean depth, a closely related factor to thermal stratification, has been suggested to modify the rate at which abiotic factors affect the phytoplankton community (Alvarez Cobelas et al. 1994) and is a key element in the development of coastal phytoplankton blooms (Koseff et al. 1993). Both thermal stratification and herbivory influence the relationship between total phosphorus and algal biomass (Mazumder 1994).

However, little is known about the relative importance and interaction of nutrient availability, herbivorous predation, and thermal structure on phytoplankton community structure. Traditionally, the study of phytoplankton community structure has been approached from either one or, at the most, from a combination of two of these three factors. For example, physical-nutrient interactions have been studied experimentally in the Blelham Tarn enclosures (Lund and Reynolds 1982) and empirically among lakes (Straskraba 1980, Pridmore et al. 1985). Nutrient-trophic interactions have received increasing attention (McQueen et al. 1986, Mazumder et al. 1988, Mazumder 1994, Carpenter et al. 1995), but most studies have examined effects on phytoplankton biomass (chlorophyll a). Fewer have quantitatively assessed the effects on phytoplankton community structure (Vanni 1987, Vanni and Findlay 1990). This experiment evaluates the simultaneous effects of chemical (nutrient loading), physical (presence and absence of thermal stratification), and biological factors (planktivorous fish predation) on phytoplankton communities in terms of cell number, biomass, size distribution, and taxonomic composition. The effects of nutrients and planktivorous fish were compared in shallow (with no hypolimnion) vs. deep (thermally stratified) enclosures of large dimension within an oligotrophic lake. More specifically we test the hypothesis that the phytoplankton response to nutrient loading and planktivorous fish stocking may differ in systems of contrasting thermal stratification.