Diatom Hotspots Driven by Western Boundary Current Instability

Abstract

The Mid-Atlantic Bight (MAB) region of the Northeast U.S. continental shelf is one of the world's most productive marine ecosystems (O'Reilly & Busch, 1984;O'Reilly et al., 1987) and is critical to regional commercial fisheries (Sherman et al., 1996). Unlike the MAB continental shelf and shelfbreak front (e.g., Ryan et al., 1999; Zhang et al., 2013), the MAB slope sea to the south is generally characterized by lower biomass (e.g., Xu et al., 2011), with summer subsurface chlorophyll (Chl) maximum layers dominated by nanoplankton (O'Reilly & Zetlin, 1998).MAB net community production is highly sensitive to ocean circulation (Friedrichs et al., 2019), but the response of the region's marine ecosystems to recent changes in northwest Atlantic circulation remains poorly constrained. Over the past two decades, the destabilization point of the Gulf Stream (GS) has shifted westward, resulting in more vigorous meandering of the GS south of the MAB (Andres, 2016). Consequently, the influence of the GS on the MAB has increased through both direct intrusion of GS water (Gawarkiewicz et al., 2012) and indirect interactions associated with more frequent GS shedding of anticyclonic warm-core rings (WCRs) (Gangopadhyay et al., 2019;Gawarkiewicz et al., 2018). To first order, increasing intrusions of GS water have been expected to decrease slope sea biological productivity (e.g., Brown et al., 1985;Zhang & Gawarkiewicz, 2015) as surface GS water is more oligotrophic than the slope (Brown et al., 1985;Olson et al., 1994). Here, we show observations from the MAB slope sea suggesting that the opposite can also occur.

Primary Productivity in the Mid-Atlantic Bight: Is the Shelf Break a Location of Enhanced Productivity?

“…Photoinhibition effects were included using the relationship determined by Behrenfeld and Falkowski (1997b). Daily productivity was derived by integrating the rates through the 1% isolume depth (Oliver et al, 2021;Smith et al, 2021). All variables of both models are listed in Table 1.…”

“…This may result from either the short duration of upwelling and its biological effects (Oliver et al, 2022), enhanced phytoplankton growth and accumulation below the depth of satellite detection, or the rapid removal of phytoplankton by zooplankton grazing (Zhang et al, 2013). Deep waters (near the 1% isolume) have been shown to be dominant features in summer offshore waters influenced by warm core ring intrusions (Oliver et al, 2021), but the importance of deep production as a general feature has not been established.…”

“…A second, related method is the vertically resolved method of estimating productivity, which again uses temperature, chlorophyll concentrations, and PAR in combination with a photosynthesis model. This allows for the primary productivity to be estimated throughout the water column at discrete depths and then integrated to give daily productivity estimates, and can be applied to different locations where highly resolved chlorophyll and temperature data are available (e.g., from closely spaced CTD measurements; Oliver et al, 2021;Smith et al, 2021). Such a method allows for the incorporation of small-scale features (e.g., narrow chlorophyll maxima) to be included in estimates of integrated productivity (Oliver et al, 2021), as well as to more reliably account for vertical temperature changes.…”

“…This allows for the primary productivity to be estimated throughout the water column at discrete depths and then integrated to give daily productivity estimates, and can be applied to different locations where highly resolved chlorophyll and temperature data are available (e.g., from closely spaced CTD measurements; Oliver et al, 2021;Smith et al, 2021). Such a method allows for the incorporation of small-scale features (e.g., narrow chlorophyll maxima) to be included in estimates of integrated productivity (Oliver et al, 2021), as well as to more reliably account for vertical temperature changes. Various algorithms to estimate primary productivity have been developed and compared (Campbell et al, 2002).…”

The Interaction Between Warm‐Core Rings and Submarine Canyons and Its Influence on the Onshore Transport of Offshore Waters

“…Note that the edges of these tracer fields in the initial condition are smoothed out to avoid sharp interfaces. Motivated by the possibility of canyon flow bringing nutrient-rich subsurface offshore water onto the shelf and stimulating the biological production, we define the subsurface water masses, 𝐴𝐴 𝐴𝐴𝑠𝑠𝑠𝑠2 and 𝐴𝐴 𝐴𝐴𝑟𝑟2 , as waters below the 100 m depth where nutrient concentration is much higher than the surface layer (Oliver et al, 2021). To focus on the contribution of canyons on the onshore intrusion of the subsurface offshore water, we start the vertical separation of the ring and slope waters at Day 13 when the ring starts to interact with the canyon.…”