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GLOBEC-01: Patterns of Energy Flow and Utilization on Georges Bank

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July 1, 2005 through June 30, 2006

Dr. John Steele
Woods Hole Oceanographic Institution, Woods Hole, MA 02543

Program Manager: Dr. Elizabeth Turner, NOAA/COP

Related NOAA Strategic Plan Goal:
Goal 1. Protect, restore and manage the use of coastal and ocean resources through
ecosystem-based management.

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 –weeks to months – means that the open ocean paradigm of a relatively autonomous microbial loop is inadequate. But purely top-down trophic depictions are insufficient to constrain a system subject to physical forcing as well as fishing.  These processes are found on most continental shelves but are particularly important on Georges Bank in the northwest Atlantic where the weeks-to-months regime is dominant in relative area and in productivity.

We have generated budgets for the microbial food web for three physical regimes (well mixed, sporadically mixed 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 (anchored plankton) are an additional critical pathway for transfers to higher trophic levels. Estimates of production by mesozooplankton, benthic suspension feeders and deposit feeders provide input to an upper trophic food web. Diets of commercial fish populations are used to calculate food requirements in three categories, planktivores, benthivores and piscivores, for four decades or stanzas, between which there were major changes in the fish communities.

Comparisons of fish energetic requirements for plankton and benthos, in conjunction with inputs from the microbial web, indicate that we obtained reasonable agreement for the last three decades: 1973 to 2002. But for the first decade the fish food estimates were significantly less than the inputs based on the GLOBEC years 1995-1999. This decade 1963-1972 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.

8. First Accomplishment
We constructed an end-to-end budget of the Georges Bank ecosystem by balancing the output of the lower food web with fish consumption. Mass balance was achieved by accounting specifically for the ratio of new production to production recycled through the microbial loop and for advective loss off Georges Bank.

9. Second Accomplishment
Compared with previous food webs, we estimate less consumption by juvenile fish and more by carnivorous invertebrates. Surprisingly, there is no evidence for the depletion of predatory fish during the intense exploitation in the 1980s. While the cod stock was depleted, the biomass of other piscivores peaked in the 1980s. Time series are lacking for the lower trophic levels, but there is no signal of altered piscivory to trigger a trophic cascade. Instead, there is evidence of
bottom-up control through reduced nutrient input in the 1960s.

10. Third Accomplishment
Reduction of the commercially important fish stocks was accompanied by replacement with non-commercial species, maintaining the functional diversity of the ecosystem. Bottom-up control of the food web implies that recovery of the commercially important fish stocks will be accompanied by a reversal of these species shifts, as may be occurring in the most recent decade.

Steele, J. H. & Collie, J. S. (2005). Functional diversity and stability of coastal ecosystems. In: A.R. Robinson and K. Brink, The Sea, Vol. 13 (pp. 783-817), Harvard University Press, Cambridge

J. Steele, J.Collie, J. Bisagni, M. Fogarty, D. Gifford, J. Link, M. Sieracki, B. Sullivan, A. Beet, D. Mountain, E.G. Durbin, D. Palka and W. Stockhausen (submitted). Balancing end-to-end budgets for the Georges Bank ecosystem. Progress in Oceanography

Discussions with Drs Turner, Fogarty, Sissenwine, Murawski.

Papers presented at various scientific meetings

Last updated: August 19, 2008

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