Charles W. Ramcharan

Clonal variation in depth distribution of Daphnia pulex in response to predator kairomones

In this laboratory study, we explored the variability of 47 Daphnia pulex clones in migration behavior to predator (Chaoborus and fish) kairomones in 1.6 m long tubes. The preferred mean vertical distribution in control water (no predator kairomone) is diverse among clones and responses to predator kairomone are highly variable. Some migration patterns were opposite to our expectations in clones exhibiting an upward migration in response to fish kairomone. A literature comparison indicated that this is not an unusual finding. In general, more clones responded to Chaoborus than to fish kairomone, stressing the importance of Chaoborus as a predator for Daphnia in nature. Dilution of Chaoborus kairomone led to a reduced upward migration by most Daphnia pulex clones tested and adult Daphnia exhibited a strongly reduced response to Chaoborus in comparison to juvenile Daphnia. This indicates that Daphnia may be able to respond to the actual predation threat.

Trophic triangles and competition among vertebrate (Oncorhynchus nerka, Gasterosteus aculeatus) and macroinvertebrate (Neomysis mercedis) planktivores in Muriel Lake, British Columbia, Canada

ABSTRACT We investigated whether pelagic food web dynamics, expressed through a trophic triangle, could influence the potential success of whole-lake fertilization to enhance juvenile sockeye salmon growth. Muriel Lake (145 ha), located on Vancouver Island, was fertilized during 1984 with no apparent effect on juvenile sockeye growth. Unlike most sockeye nursery lakes, Muriel Lake contains a substantial population of the invertebrate zooplanktivore Neomysis mercedis. We hypothesized that competition for zooplankton prey between Neomysis and planktivorous fish (juvenile sockeye, threespine stickleback) could counteract beneficial effects of either natural or fertilizer-induced increases in food for fish. To test this, we assessed (1985–1986) biomass, production, and consumption of planktivorous mysids and fish and then used bioenergetics models to quantify potential competitive interactions. Our analysis suggested that N. mercedis consumed 7–8% of the zooplankton standing stock per day, while planktivorous fish consumed <1.0%·d−1. Although mysids were the main consumers of zooplankton, mysids were also consumed by fish. Late in 1986, an increase in mysid consumption by large, 1+ juvenile sockeye precipitated substantial declines in Neomysis biomass. Although this event came too late to reduce mysid competition with sockeye fry in Muriel Lake in 1986, it did highlight the potential importance of trophic triangles in pelagic food webs. We suggest that mysids may be held in check by juvenile sockeye when exogenous recruitment events result in high sockeye biomass. By contrast, recruitment failures and low sockeye biomass promote increases in Neomysis populations, which then control zooplankton communities such that sockeye gain little benefit from either natural or fertilizer-induced increases of zooplankton. For food web ecologists, the implication is that trophic triangles might produce alternate stable states that are mediated by external factors. For fisheries managers, the implication is that lakes containing mysids should only be fertilized when sockeye densities are high or mysid densities are low.

Biomass of benthic invertebrates unaffected by industrial damage to lakes despite effects on species composition

Historic metal mining and smelting in the area of Sudbury, Ontario, has resulted in lakes with elevated levels of metals in the sediment, low inputs of terrestrial organic material, and unusual fish communities which led to severe changes in the benthic invertebrate community. Using broad-scale surveys of lakes from Sudbury and an undisturbed reference area, we used multivariate methods to tease apart potential influences of predation by fish, habitat availability, and toxicity of the environment on the biomass of benthic invertebrates. In contrast to many other studies, our response variable is benthic invertebrate biomass—a more relevant factor in food web processes than the numeric densities that are more typically studied. Despite differences in species composition, we observed very similar total benthic invertebrate biomass in Sudbury lakes and reference lakes. Redundancy analysis revealed that different factors shaped invertebrate biomass in the two study areas. Sudbury benthos was strongly influenced by the fish community and metal toxicity with only slight effects of habitat quality. Overall, the fact that benthic invertebrate biomass was similar to reference systems suggests that Sudbury communities may be able to function normally despite the strong direct and indirect effects of multiple stressors.