Water masses and nutrient sources to the Gulf of Maine

Abstract

The Gulf of Maine, a semi-enclosed basin on the continental shelf of the northwest Atlantic Ocean, is fed by surface and deep water flows from outside the Gulf: Scotian Shelf Water from the Nova Scotian shelf that enters the Gulf at the surface, and Slope Water that enters at depth and along the bottom through the Northeast Channel. There are two types of Slope Water, Labrador Slope Water (LSW) and Warm Slope Water (WSW); it is these deep water masses that are the major source of dissolved inorganic nutrients to the Gulf. It has been known for some time that the volume inflow of Slope Waters of either type that enters the Gulf of Maine is variable, that it co-varies with the magnitude of inflowing Scotian Shelf Water, and that periods of greater inflows of Scotian Shelf Water have become more frequent in recent years, accompanied by reduced Slope Water inflows. We present here analyses of a ten-year record of data collected by moored sensors in Jordan Basin, in the interior Gulf of Maine, and in the Northeast Channel, along with recent and historical hydrographic and nutrient data, that help reveal the nature of Scotian Shelf Water and Slope Water inflows. Proportional inflows of nutrient-rich Slope Waters and nutrient-poor Scotian Shelf Waters alternate episodically with one another on time scales of months to several years, creating a variable nutrient field upon which the biological productivities of the Gulf of Maine and Georges Bank depend. Unlike decades past, the inflows of Slope Waters of either type do not appear to be correlated with the North Atlantic Oscillation, which had been shown earlier to influence the relative proportions of the two Slope Waters, WSW and LSW, that enter the Gulf. We suggest that of greater importance in recent years are more frequent, episodic influxes of colder, fresher, less dense, and low-nutrient Scotian Shelf Water into the Gulf of Maine, and concomitant reductions in the inflow of deep, nutrient-rich Slope Waters. We also discuss evidence of modified Gulf Stream ring water that penetrated to Jordan Basin in summer of 2013.

Gulf of Maine salinity variation and its correlation with upstream Scotian Shelf currents at seasonal and interannual time scales

“…As discussed most recently in Townsend et al . [], one key puzzle in diagnosing variable Gulf hydrography lies in the complexity of the sea‐slope exchange within the NEC, which in a given year, or even month, can involve an inflow admixture of Atlantic slope water, Labrador slope water, Gulf Stream meanders, and Scotian Shelf water. NEC inflow and outflow is now known to be highly variable with depth [ Ramp et al ., ; Smith et al ., ] as well as across and along the NEC, making it a difficult system to monitor and evaluate.…”

“…There is significant recent focus on the identification and explanation of ecological, biochemical, and physical change at seasonal to decadal time scales in the Gulf of Maine (GoM) and surrounding waters including the Nova Scotian Shelf (SS) [ Mountain , ; Pershing et al ., ; Smith et al ., ; Ji et al ., ; Townsend et al ., ]. Interest in GoM water mass variability is not new [ Bigelow , ; Fournier et al ., ; Petrie and Drinkwater , ; Loder et al ., ] with prevailing reasons being the presence of extensive marine life and active fisheries.…”

“…[] and Townsend et al . [] suggest the likelihood of net balance and feedbacks between these inflows; increased Cape Sable SS inflow is postulated to inhibit deepwater NEC inflow. Their work also suggests new emphasis be placed on detecting and understanding variability in this northern channel inflow pathway at interannual to decadal scales.…”

Developing the First Operational Nutrient Observatory for Ecosystem, Climate, and Hazard Monitoring for NERACOOS

“…Typically, however, the source of nutrients that stimulates these toxic blooms is overwhelmingly dominated by the ocean end member (Townsend, 1998;Anderson et al, 2008). Most critical for this project is that there have been significant changes in recent years to the water properties and inorganic nutrient loads of the deep offshore-derived water masses in the GOM (e.g., Pettigrew et al, 2008;Rebuck, 2011;Smith et al, 2012;Townsend et al, submitted) that need to be better understood and monitored. showed that deep waters in the GOM (>100 m) have become slightly fresher and cooler since the 1970s, with lower nitrate (by~2-4 μM) but higher silicate (also bỹ 2-4 μM).…”

Assessing Coastal SMAP Surface Salinity Accuracy and Its Application to Monitoring Gulf of Maine Circulation Dynamics

“…Key water mass inflows influencing this marginal sea include the upstream fresh/cold Scotian Shelf and Labrador Sea currents, and the adjoining salty/warm northwestern (NW) Atlantic shelf (Mountain [1]; Townsend et al [2]; Feng et al [3]; Peterson et al [4]). The primary inflow to the GoM is through The dominance of subsurface GoM exchange pathways has, so far, limited the utility of surface remote sensing data for monitoring GoM dynamics.…”