In marine sciences, a
bottom-up trophic control refers to an ecosystem that is resource-driven and limited by biotic or non-biotic parameters, such as the physical environment and secondary production, whereas its counterpart, a top-down
trophic control, refers to a consumer-driven cascade where the dominant control is exerted by predators such as fish and marine mammals. In my keynote at the Capelin Symposium Bergen, I presented evidence of climate variability exerting a bottom-up control on the fish community of the Canadian Newfoundland and Labrador (NL) shelf, a region known as an
iconic fishing ground for centuries and also
prone to large climate variability.
Located at the confluence of Arctic, Subarctic, and subtropical currents, the NL shelf is especially affected by large-scale ocean circulation changes. Such circulation changes impact not only the regional ocean climate but also the overall composition of water masses and the immediate habitat of numerous commercial and non-commercial fish species.
Canada and other countries carried out ocean observations over the last seven decades. Using these observations, the time period was separated into different phases (or regimes) based on the trends in the region's mean climate. For example, the period 1948–1971 was found to be the warmest period in recent history, while the decade between the mid-1980s and mid-1990s was the coldest - a period that also witnessed a near-complete collapse of most fisheries in the region.
Climate regimes
Our findings show that these different climate regimes are well explained by changes in atmospheric pressure at sea level. These changes, accompanied by modifications in the mean wind fields, also affect large-scale ocean circulation such as the strength of the subpolar gyre, and thus the interactions between the Labrador and the North Atlantic currents, two major contributors to the NL shelf climate variability. Furthermore, the different climate regimes on the NL shelf are linked to changes in the productivity of the ecosystem and the natural variability in fish biomass, including groundfish species, but also capelin, the keystone forage species in the region.
The following hypotheses explain these findings and are well supported by observations: during warmer climatic phases, the spring phytoplankton bloom occurs earlier and may better match the emergence of key zooplankton species (e.g. Calanus finmarchicus) from overwintering. This further translates into increased food availability for capelin, and thus more efficient energy transfer from primary and secondary producers to higher trophic levels. In turn, this explains the observed natural fluctuations (non-fishery related) of capelin and other fish biomasses.
During colder phases of the NL climate, the phytoplankton bloom is later, which may result in a mismatch with Calanus emergence from overwintering and, consequently, less food availability for capelin and other fish species. Since these climate changes occur at a relatively low frequency (decadal timescales), the recognition of the current ecosystem phase (or regime) offers a potential forecasting capability of ecosystem productivity that could be integrated into fish stock assessments.
The Capelin Symposium Bergen continues all this week. Follow the news coming from Bergen @CSB_2022.
