Scotian shelf (Eastern) - Ecology

The dynamics (timing and magnitude) of the principal phytoplankton grazers (zooplankton) will determine the extent to which phytoplankton production is transferred to the pelagic food chain or goes directly, as phyto-detritus, to the benthos. Although the phytoplankton and mesozooplankton communities cannot be managed, it is important to recognise that changes in response to variable hydrographic conditions can occur and that they can affect higher trophic levels through bottom-up processes. Disruptions in the normal timing of the phytoplankton bloom and Calanus finmarchicus reproductive and developmental cycles may have serious consequences for the survival of pre-settled groundfish and may influence the dynamics of subsequent recruitment. These sorts of changes cannot be controlled and must be recognized in the development and implementation of integrated management schemes.
There is ample evidence that variability in the physical environment and lower trophic levels, variability in the abundance of high-level predators, and human activities can have significant and long-lasting effects on marine ecosystems. The eastern Scotian Shelf ecosystem has been profoundly altered and exhibits classic symptoms a trophic cascade and of “fishing down the foodweb“. Comparison of two Ecopath models indicated that although total productivity and total biomass of the ecosystem remained similar between the early 1980s and late 1990s, there were changes in predator structure, trophic structure and energy flow, many of which were robust to uncertainty. There was a significant decrease in the biomass of demersal groundfish species and a significant increase in biomass of grey seals, small pelagic species, commercial crustaceans, and phytoplankton. The greatest change in the ecosystem is the switch from a demersal (D) dominated system to a pelagic (P) dominated system, indicating a shift in trophic flow from the demersal to the pelagic side of the food web. The P:D ratio is an indicator of the negative effects of fishing.
Changes in biodiversity, manifested either as reduced species richness or changes in the proportional composition of the component species, can result in complex reorganizations of ecosystems such as we have seen on the eastern Scotian Shelf. Such reorganizations exhibit changes in production and stability and are mediated through changes in trophic interactions that can result in trophic cascades and shifts to undesirable states of the ecosystem. These reorganizations in turn have impacts on the richness and particular species composition of the reorganized ecosystem. Thus feedback mechanisms initiated by changes in biodiversity can cause further changes in biodiversity. The eastern Scotian Shelf has switched to an alternative state, dominated by small pelagics, invertebrates and seals. This state of hysteresis, whereby the previously abundant cod are kept at low levels of abundance, is maintained through a combination of cultivation-depensation effects, Allee effects and a massive reduction in the size structure of what remains of the cod population.