Mountain David G.

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Mountain
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David G.
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  • Article
    Remote climate forcing of decadal-scale regime shifts in Northwest Atlantic shelf ecosystems
    (Association for the Sciences of Limnology and Oceanography, 2013-05) Greene, Charles H. ; Meyer-Gutbrod, Erin ; Monger, Bruce C. ; McGarry, Louise P. ; Pershing, Andrew J. ; Belkin, Igor M. ; Fratantoni, Paula S. ; Mountain, David G. ; Pickart, Robert S. ; Proshutinsky, Andrey ; Ji, Rubao ; Bisagni, James J. ; Hakkinen, Sirpa M. A. ; Haidvogel, Dale B. ; Wang, Jia ; Head, Erica ; Smith, Peter ; Reid, Philip C. ; Conversi, Alessandra
    Decadal-scale regime shifts in Northwest Atlantic shelf ecosystems can be remotely forced by climate-associated atmosphere–ocean interactions in the North Atlantic and Arctic Ocean Basins. This remote climate forcing is mediated primarily by basin- and hemispheric-scale changes in ocean circulation. We review and synthesize results from process-oriented field studies and retrospective analyses of time-series data to document the linkages between climate, ocean circulation, and ecosystem dynamics. Bottom-up forcing associated with climate plays a prominent role in the dynamics of these ecosystems, comparable in importance to that of top-down forcing associated with commercial fishing. A broad perspective, one encompassing the effects of basin- and hemispheric-scale climate processes on marine ecosystems, will be critical to the sustainable management of marine living resources in the Northwest Atlantic.
  • Preprint
    Balancing end-to-end budgets of the Georges Bank ecosystem
    ( 2007-05-09) Steele, John H. ; Collie, Jeremy S. ; Bisagni, James J. ; Gifford, Dian J. ; Fogarty, Michael J. ; Link, Jason S. ; Sullivan, B. K. ; Sieracki, Michael E. ; Beet, Andrew R. ; Mountain, David G. ; Durbin, Edward G. ; Palka, D. ; Stockhausen, W. T.
    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 pelagic-benthic 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 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, 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.
  • Article
    Influence of ocean freshening on shelf phytoplankton dynamics
    (American Geophysical Union, 2007-12-28) Ji, Rubao ; Davis, Cabell S. ; Chen, Changsheng ; Townsend, David W. ; Mountain, David G. ; Beardsley, Robert C.
    Climate change-induced freshening of the ocean can enhance vertical stratification and alter circulation patterns in ways that influence phytoplankton dynamics. We examined the timing of spring phytoplankton blooms and the magnitude of net primary productivity in the Nova Scotian Shelf (NSS) - Gulf of Maine (GoM) region with respect to seasonal and interannual changes in surface water freshening from 1998 to 2006. The general pattern of temporal westward progression of the phytoplankton bloom corresponds with the gradient of increasing sea surface salinity from the NSS in the east to the western GoM. Increased freshening enhances the spatial gradients in bloom timing by stimulating earlier blooms upstream (NSS), but it has less impact downstream (the western GoM). Strong spatial gradients (increasing westward) of mean chlorophyll concentration and net primary productivity during post-bloom months (May–June) indicate that lower sea surface salinity upstream can likely impede nutrient fluxes from deep water and therefore affect overall productivity.
  • Article
    Suppression of the 2010 Alexandrium fundyense bloom by changes in physical, biological, and chemical properties of the Gulf of Maine
    (Association for the Sciences of Limnology and Oceanography, 2011-11) McGillicuddy, Dennis J. ; Townsend, David W. ; He, Ruoying ; Keafer, Bruce A. ; Kleindinst, Judith L. ; Li, Y. ; Manning, James P. ; Mountain, David G. ; Thomas, Maura A. ; Anderson, Donald M.
    For the period 2005–2009, the abundance of resting cysts in bottom sediments from the preceding autumn was a first-order predictor of the overall severity of spring–summer blooms of Alexandrium fundyense in the western Gulf of Maine and southern New England. Cyst abundance off mid-coast Maine was significantly higher in autumn 2009 than it was preceding a major regional bloom in 2005. A seasonal ensemble forecast was computed using a range of forcing conditions for the period 2004–2009, suggesting that a large bloom was likely in the western Gulf of Maine in 2010. This did not materialize, perhaps because environmental conditions in spring–summer 2010 were not favorable for growth of A. fundyense. Water mass anomalies indicate a regional-scale change in circulation with direct influence on A. fundyense's niche. Specifically, near-surface waters were warmer, fresher, more stratified, and had lower nutrients than during the period of observations used to construct the ensemble forecast. Moreover, a weaker-than-normal coastal current lessened A. fundyense transport into the western Gulf of Maine and Massachusetts Bay. Satellite ocean color observations indicate the 2010 spring phytoplankton bloom was more intense than usual. Early season nutrient depletion may have caused a temporal mismatch with A. fundyense's endogenous clock that regulates the timing of cyst germination. These findings highlight the difficulties of ecological forecasting in a changing oceanographic environment, and underscore the need for a sustained observational network to drive such forecasts.
  • Article
    Recent Arctic climate change and its remote forcing of Northwest Atlantic shelf ecosystems
    (The Oceanography Society, 2012-09) Greene, Charles H. ; Monger, Bruce C. ; McGarry, Louise P. ; Connelly, Matthew D. ; Schnepf, Neesha R. ; Pershing, Andrew J. ; Belkin, Igor M. ; Fratantoni, Paula S. ; Mountain, David G. ; Pickart, Robert S. ; Ji, Rubao ; Bisagni, James J. ; Chen, Changsheng ; Hakkinen, Sirpa M. A. ; Haidvogel, Dale B. ; Wang, Jia ; Head, Erica ; Smith, Peter ; Conversi, Alessandra
    During recent decades, historically unprecedented changes have been observed in the Arctic as climate warming has increased precipitation, river discharge, and glacial as well as sea-ice melting. Additionally, shifts in the Arctic's atmospheric pressure field have altered surface winds, ocean circulation, and freshwater storage in the Beaufort Gyre. These processes have resulted in variable patterns of freshwater export from the Arctic Ocean, including the emergence of great salinity anomalies propagating throughout the North Atlantic. Here, we link these variable patterns of freshwater export from the Arctic Ocean to the regime shifts observed in Northwest Atlantic shelf ecosystems. Specifically, we hypothesize that the corresponding salinity anomalies, both negative and positive, alter the timing and extent of water-column stratification, thereby impacting the production and seasonal cycles of phytoplankton, zooplankton, and higher-trophic-level consumers. Should this hypothesis hold up to critical evaluation, it has the potential to fundamentally alter our current understanding of the processes forcing the dynamics of Northwest Atlantic shelf ecosystems.