Determination of Colpoys Bay (Georgian Bay) benthic community trophic structure and energy flow using stable isotopes and secondary production

Abstract

In this thesis the energy flow through the benthic food web of an oligotrophic lentic system, Coploys Bay, in Georgian Bay was described. Abundance, biomass, secondary production, species richness and diet of invertebrates in three contrasting habitats along a depth gradient were compared. Estimates of diet, obtained using stable isotopes of carbon and nitrogen and secondary production were combined to determine the dependence of each benthic community on autochtonous littoral versus pelagic sources and/or allochthonous inputs.

Among the three main zones studied, animals occupying charophyte beds make the most significant contribution (51.8%) to the total production of the bay. The site within the shallow littoral zone was more productive on a per unit area, but this habitat is found in less of the basin, so it accounted for slightly less secondary production than the charophyte bed. The profundal zone is largest in terms of area but supports a more restricted fauna and contributed only 9% of the benthic secondary production. The drastic decline at deeper depths (i.e. below the photic zone) suggests that benthic primary production determines the secondary production of Colpoys Bay. This study emphasized that a significant proportion of energy transfer between primary production and fish is through the benthic food chain.

The linkages between dissolved inorganic carbon (DIC) and particulate organic matter (POM) were investigated in the pelagic zone of Colpoys Bay. The results suggest that the temporal fluctuation in pelagic POM 8^13C is influenced by algal species changes in response to changes in the type (CO2 or HCO3) of aquatic DIC and seasonal intrusion of littoral matter. Inferences on the variation observed in the zooplanktonic community were also made. The general trend in the isotope signatures of zooplankton followed those of POM.

The trophic structure of macroinvertebrate communities in the littoral (<_ 15 m) and profundal (>_ 30 m) zones was assessed using stable isotopes of carbon and nitrogen. Energy sources included periphyton, macrophytes, POM and allochtonous organic matter. Spatial and temporal isotopic variation were reported at the primary producer level and consequently primary consumers. Periphyton was the main energy source for benthic communities within littoral areas of Colpoys Bay. Littoral macrophytes, macroalga and allochothonous matter are not used as energy sources for invertebrates. Regardless of what feeding category an invertebrate was assigned, the vast majority of them are strongly dependent on the epilithic biofilm for food supply. This dependence on the epilithic biofilm diminishes with increasing water depth. Invertebrates at the sub-littoral site relied on a combination of epilithon, epiphyton and POM; those in the profundal region were dependent on autochthonous pelagic organic matter.

In order to assess the trophic role of Diporeia hoyi in Colpoys Bay, amphipods were collected depths of 30 and 50 m These were used to examine the life cycle dynamics, estimate production, stomach fullness, lipid content and stable isotopes of carbon and nitrogen. In agreement with previous studies, D.hoyi collected in areas deeper than 30 m seem to rely on pelagic primary production for energy sources as shown by increased feeding activity during spring followed by increased lipid content. Stable carbon isotopes showed that D.hoyi will also use littoral epilithon when it is available.