Trophic manipulation in freshwater plankton communities

Abstract

The effects of zooplankton herbivory on natural phytoplankton communities in two urban mesotrophic lakes in the North Island of New Zealand were studied over the course of one year. Lakes Rotoroa and Pupuke have zooplankton assemblages typical of most New Zealand lakes, in having a low species diversity, being dominated by calanoid copepods, and lacking Daphnia. In situ enclosure (limnocorral) experiments were conducted in each season of the year in each lake using 1 m³ (1000 litre) volume plastic bags. The enclosures were initially filled with lake water filtered through an 80 μm mesh plankton net to remove the majority of the zooplankton. Subsequently, a gradient of zooplankton biomass was established in eight enclosures by adding crustacean zooplankton collected from the epilimnion. Four densities (each duplicated) were established: exclosure (no zooplankton added), 1X, 2X, and 4X ambient zooplankton biomass. Experiments were conducted for 10 days with physico-chemical and biological sampling made on Days 0, 3, 6, and 10. Their aim was to investigate the role of zooplankton grazing on total algal biomass, the dynamics of population level responses of the phytoplankters, feeding selectivities of the herbivores and any changes in nutrient dynamics mediated by zooplankton grazing. In Lake Rotoroa zooplankton grazing had no significant effect on total algal biomass in autumn. In winter, spring and summer there were significant reductions in phytoplankton biomass particularly in the size fraction ≤ 80 μm. In Lake Pupuke grazing caused no change in algal biomass in autumn and winter, but in spring it was significantly reduced. However, in summer algal biomass was positively associated with zooplankton biomass possibly indicating that nutrient regeneration by zooplankters was important in this season. At the population level, algal species-specific response to herbivory varied between grazing suppression, a lack of response, increased abundance or in some cases, a unimodal response. The majority of phytoplankters in Lake Rotoroa were ‘decreasers’. In Lake Pupuke most algae were ‘unresponsive’ to grazing. Most ‘decreasers’ were of smaller size ≤ 30 μm in Greatest Axial Linear Dimension (GALD). ‘Unresponsive’ algae and ‘increasers’ were usually >30 μm in GALD. Relationships with algal biovolume were less consistent, but there was an upper limit to the volume of suppressed algae (~ 6,000 μm³) above which no species were significantly grazed. In Lake Rotoroa the presence of herbivores increased the availability of nitrogen (NH₄ and DON) in spring and of phosphorus (DRP and DOP) in summer. In Lake Pupuke zooplankton altered nutrient concentrations little during spring but increased the availability of both organic N and P during the summer experiment. The results suggest that integration of the ecological forces of grazing and nutrient limitation are required to explain patterns of planktonic seasonal succession, rather than depicting the two control processes as mutually exclusive.