of introduced species and harmful or nuisance organisms are increasing
(Smayda 1990, Hallegraeff 1993, Anderson 1995).
On shelf-wide scales, it is recognized that the coastal northwest Atlantic
is highly productive and plays an active role in the regional and global
cycling of carbon and other elements (e.g., O'Reilly et al. 1987, Walsh et al.
1987). The limiting nutrient for phytoplankton production in the continental
shelf ecosystems of the North Atlantic is nitrogen, in part due to significant
benthic denitrification that results in a major loss from this system to N 2 gas
(Seitzinger 1988, Seitzinger and Giblin 1996, Devol and Christensen 1993).
In the Mid-Atlantic Bight (MAB), this loss is estimated to exceed the input
of nitrogen from land and atmospheric sources. The high primary
productivity is sustained by added nitrogen inputs from “onwelling” –
various shelf-sea/deep-ocean exchange circulation processes that produce
net onshore transport of new nutrients derived, ultimately, from regeneration
in the deep ocean (Seitzinger 1988). Calculating a nitrogen budget for the
MAB shelf therefore demands consideration of both biogeochemical
processes, such as denitrification and primary production, but equally
physical circulation and mixing.
Observational efforts to quantify shelf-sea/open-ocean exchange of
nutrients and carbon include the Shelf Edge Exchange Processes (SEEP)
experiments I and II (from 1983 to 1989) and the Ocean Margins Program
(OMP) experiment (from 1994 to 1996). Results from SEEP showed that in
the northern MAB only a small proportion of the particulate organic carbon
(POC) produced on the shelf is exported while most of it is recycled by
consumption or oxidized on the shelf (Anderson et al. 1994, Biscaye et al.
1994, Falkowski et al. 1988, Walsh 1994). However, the SEEP study
assumed that carbon flux within the coastal ocean, and export off the shelf to
deeper waters, is dominated by POC (typically measuring the larger sinking
fraction of POC). The OMP experiment explicitly included contributions
from dissolved organic and inorganic carbon (DOC, DIC) and suspended
POC (Verity et al. 2002). We now know that the DOC pool in the MAB is
one to three orders of magnitude larger than the POC pool (Bauer et al.
2001) and, being dissolved, is therefore readily transported by ocean
circulation.
The salinity of shelf waters in the MAB and further north is significantly
lower (3 to 7 psu) than adjacent open ocean waters due to the trapping of the
circulation on the shelf by the shelf-slope front (Chapman and Beardsley,
1989). Significant inter-annual variations in this shelf water salinity, and to a
lesser extent temperature, have been documented in the MAB by Mountain
(2003). These anomalies have their origin in circulation processes acting
outside the MAB, entering the region via the Gulf of Maine but being
ultimately driven further upstream on the Scotian shelf or beyond in the
Labrador Sea. As occurs for salinity, strong gradients in dissolved organic
matter (DOM) concentration exist between coastal waters and the open
ocean (Hopkinson et al. 2002). Mixing of shelf and slope waters by physical
FORECAST AND MODELS FOR COASTAL OBSERVATORIES 551
(Smayda 1990, Hallegraeff 1993, Anderson 1995).
On shelf-wide scales, it is recognized that the coastal northwest Atlantic
is highly productive and plays an active role in the regional and global
cycling of carbon and other elements (e.g., O'Reilly et al. 1987, Walsh et al.
1987). The limiting nutrient for phytoplankton production in the continental
shelf ecosystems of the North Atlantic is nitrogen, in part due to significant
benthic denitrification that results in a major loss from this system to N 2 gas
(Seitzinger 1988, Seitzinger and Giblin 1996, Devol and Christensen 1993).
In the Mid-Atlantic Bight (MAB), this loss is estimated to exceed the input
of nitrogen from land and atmospheric sources. The high primary
productivity is sustained by added nitrogen inputs from “onwelling” –
various shelf-sea/deep-ocean exchange circulation processes that produce
net onshore transport of new nutrients derived, ultimately, from regeneration
in the deep ocean (Seitzinger 1988). Calculating a nitrogen budget for the
MAB shelf therefore demands consideration of both biogeochemical
processes, such as denitrification and primary production, but equally
physical circulation and mixing.
Observational efforts to quantify shelf-sea/open-ocean exchange of
nutrients and carbon include the Shelf Edge Exchange Processes (SEEP)
experiments I and II (from 1983 to 1989) and the Ocean Margins Program
(OMP) experiment (from 1994 to 1996). Results from SEEP showed that in
the northern MAB only a small proportion of the particulate organic carbon
(POC) produced on the shelf is exported while most of it is recycled by
consumption or oxidized on the shelf (Anderson et al. 1994, Biscaye et al.
1994, Falkowski et al. 1988, Walsh 1994). However, the SEEP study
assumed that carbon flux within the coastal ocean, and export off the shelf to
deeper waters, is dominated by POC (typically measuring the larger sinking
fraction of POC). The OMP experiment explicitly included contributions
from dissolved organic and inorganic carbon (DOC, DIC) and suspended
POC (Verity et al. 2002). We now know that the DOC pool in the MAB is
one to three orders of magnitude larger than the POC pool (Bauer et al.
2001) and, being dissolved, is therefore readily transported by ocean
circulation.
The salinity of shelf waters in the MAB and further north is significantly
lower (3 to 7 psu) than adjacent open ocean waters due to the trapping of the
circulation on the shelf by the shelf-slope front (Chapman and Beardsley,
1989). Significant inter-annual variations in this shelf water salinity, and to a
lesser extent temperature, have been documented in the MAB by Mountain
(2003). These anomalies have their origin in circulation processes acting
outside the MAB, entering the region via the Gulf of Maine but being
ultimately driven further upstream on the Scotian shelf or beyond in the
Labrador Sea. As occurs for salinity, strong gradients in dissolved organic
matter (DOM) concentration exist between coastal waters and the open
ocean (Hopkinson et al. 2002). Mixing of shelf and slope waters by physical
FORECAST AND MODELS FOR COASTAL OBSERVATORIES 551