Balancing end-to-end budgets of the Georges Bank ecosystem

Abstract

Oceanographic regimes on the continental shelf display a great range in the time scales of physical exchange, biochemical processes and trophic transfers. The close surface-to-seabed physical coupling at intermediate scales of weeks to months means that the open ocean simplification to a purely pelagic food web is inadequate. Top-down trophic depictions, starting from the fish populations, are insufficient to constrain a system involving extensive nutrient recycling at lower trophic levels and subject to physical forcing as well as fishing. These pelagicbenthic interactions are found on all continental shelves but are particularly important on the relatively shallow Georges Bank in the northwest Atlantic. We have generated budgets for the 1 lower food web for three physical regimes (well mixed, transitional and stratified) and for three seasons (spring, summer and fall/winter). The calculations show that vertical mixing and lateral exchange between the three regimes are important for zooplankton production as well as for nutrient input. Benthic suspension feeders are an additional critical pathway for transfers to higher trophic levels. Estimates of production by mesozooplankton, benthic suspension feeders and deposit feeders, derived primarily from data collected during the GLOBEC years of 1995-1999, provide input to an upper food web. Diets of commercial fish populations are used to calculate food requirements in three fish categories, planktivores, benthivores and piscivores, for four decades, 1963-2002, between which there were major changes in the fish communities.Comparisons of inputs from the lower web with fish energetic requirements for plankton and benthos indicate that we obtained reasonable agreement for the last three decades, 1973 to 2002.However, for the first decade, the fish food requirements were significantly less than the inputs. This decade, 1963This decade, -1972, corresponds to a period characterized by a strong Labrador Current and lower nitrate levels at the shelf edge, demonstrating how strong bottom-up physical forcing may determine overall fish yields.

Constructing end-to-end models using ECOPATH data

“…An E2E web must also take account of (v) the smaller space and time scales that are important for the microbial food web. In general, the calculation of annual averages for these lower trophic levels over the whole ecosystem may require initial subdivision into smaller spatial and shorter temporal components to capture the topography and seasonal cycling of the lower planktonic web (e.g., Steele et al, 2007); or construction of NPZD models that are then used for annual averages (Aydin et al, 2007). Choice of method will depend on the nature of the system and the data available.…”

“…1. For large semi-enclosed systems such as the North Sea, input/output fluxes may be less significant than for smaller offshore regions such as Georges Bank (Heath and Beare, 2009;Steele et al, 2007). Overall, varying flux rates can be expected to be a defining feature in the comparison of shelf ecosystems.…”

“…Carbon is an alternative unit for both systems and will be used here. As scaling factors we use the Redfield ratio (6.625 mmole C mmole N -1 and the estimated carbon content of fish on Georges Bank (8.8 mg live weight mg C -1 ) (Steele, 2007).…”

“…In a comparable Georges Bank study (Steele et al, 2007) the lower food web fluxes were determined for three regions on the Bank and for three seasons. More complex models such as ERSEM (Baretta et al, 1995) or ATLANTIS (Fulton and Smith, 2004) use regional oceanographic models.…”

Integrated ecological–economic fisheries models—Evaluation, review and challenges for implementation

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