nutrient enrichment at a station about 2 miles (3 km) south of the estuary 

 (Garside, unpublished ) . Nevertheless, estuaries undoubtedly do provide some 

 enhancement of the nutrient supply in their immediate vicinity. 



A second factor that has recently been recognized as important in resupplying 

 nutrients to the surface layer is storms (Walsh et al. 1978). Turbulent 

 mixing associated with storms (see "Hydrography," above) causes mixing of 

 surface waters and is most effective in resupplying nutrients when the 

 thermocline is shallow, shortly after its formation. The timing of the fall 

 overturn, which brings nutrients into surface waters, may be influenced by 

 storms . 



Upwelling and the interaction of tidal currents with the sea floor also 

 produce turbulent energy that can cause mixing across the thermocline. The 

 high nutrient concentrations found throughout the summer from Mt . Desert 

 (region 5) to the Bay of Fundy (region 6; Apollonio and Applin 1972) may be 

 maintained by these processes. A tongue of high nitrate water extends down 

 the coast, with maximum values of 7 yM NO3 close to the Bay of Fundy and 10 

 to 15 miles (16 to 24 km) offshore; with declining values inshore (to 4 yM) 

 and offshore, as well as to the southwest, to <1 uM off Penobscot Bay (region 

 4). 



Over the area within which upwelling seems to be taking place (regions 5 and 



6), nutrients are not usually limiting to phytoplankton productivity. The 



potential production of such water is probably equivalent at least to the 



Sheepscot Estuary (150gm y~^ ; Garside et al. 1978) for a net yield of 1.3 



million tons (1.8 million metric tons) of carbon fixed per year. The 



importance of such productivity to the Gulf of Maine fisheries cannot be 



ignored, but, unfortunately, little is known of the mechanisms influencing the 

 nutrient supply that supports it. 



Where depths are sufficiently shallow, the thermocline may reach the bottom. 

 When this occurs, nutrients regenerated in the sediments can be released to 

 the surface layer and become available for reuse. It may not be possible to 

 detect an increased nutrient concentration, since nutrients often are used as 

 fast as they are supplied, but productivity is increased as a consequence of 

 the process. Such processes are restricted to water depths of 66 to 100 feet 

 (20 to 30 m) . Where these depths are found over extensive areas (such as 

 Georges Bank) , this regeneration can contribute significantly to maintaining 

 high levels of productivity. 



Organic matter cycle . Nearly all organic matter found in the natural 

 environment has been derived from, or is a constituent of, living organisms. 

 Since living organisms interact closely with abiotic organic matter in their 

 environment, metabolically active compounds are passed rapidly from species to 

 species, during feeding, assimilation, growth, and death. Other organic 

 components include toxic substances that may profoundly affect growth in 

 minute concentrations, and relatively inert compounds that may be transformed 

 to active compounds by abiotic factors (e.g., wave action and salinity 

 changes) . 



To illustrate the potential significance of dissolved organic substances in 

 the oceans when compared to the sizes of living organisms, the organic carbon 

 content of an open ocean water sample is plotted in figure 4-17 as a function 



4-50 



