Effects of Nutrients and Planktivorous Fish on the Phytoplankton of Shallow and Deep Aquatic Systems

Abstract

The response of phytoplankton to nutrient (N, P) and planktivorous fish additions (Phoxinus sp.) was compared between two sets of enclosures (8 m in diameter) installed at different depths in a dimictic temperate lake. One set of eight enclosures was located in the deep part of the lake (12 m) where water thermally stratifies during the summer. The other set of eight enclosures was located in a shallow area of the lake (3.5 m) where the water does not thermally stratify. During the summer months, phytoplankton cell number and biomass increased significantly with both nutrient additions (P = 0.004) and the presence of planktivorous fish (P = 0.011) in the deep, thermally stratified enclosures. On average, total algal biomass in the nutrient and fish treatments was 2.6 times higher than in the nutrient—only treatments. In contrast, in the shallow enclosures, total phytoplankton biomass increased with nutrient additions (P = 0.04) but was not significantly affected by the addition of fish (P = 0.34). However, fish additions increased the number of algal cells in both the shallow and deep enclosures, independently of nutrients, because of an increase in small cells: the proportion of nanoplankton cells was significantly greater in the presence of fish. Fertilization increased the large nanoplankton fraction (10—20 μm) in the deep enclosures, but had no significant effect on the size distribution of algal biomass in the shallow enclosures. In general, large (> 64 μ m) cells were a minor component of the phytoplankton biomass in all treatments. Fertilization stimulated the growth of Chlorophyta in the deep (P = 0.018) and in the shallow (P = 0.012) enclosures. Cyanobacteria were conspicuous in the fertilized shallow enclosures, but the effect on total biomass was not statistically significant. The presence of fish stimulated a significant increase in Cryptophyta at both depths and Pyrrophyta were more common. Nutrients and planktivorous fish had different effects on phytoplankton depending on the thermal regime. In terms of total algal biomass there was no significant difference between deep and shallow enclosures within a given treatment, but depth did affect the size distribution of algal biomass among treatments. The effects of fish were more evident with community structure variables (size or taxonomic distribution) than with coarser variables such as algal biomass.