ging source waters on the Northeast US Continental Shelf: Variation in nutrient supply and phytoplankton biomass

Authors: FREDLAND, KEVIN - Southwest Fisheries Science Center; FRATANTONI, PAULA - Southwest Fisheries Science Center; SILVER, ADRIENNE - Woods Hole Oceanographic Institute (WHOI); BRADY, DAMIAN - University Of Maine; GANGOPADHYAY, AVIJIT - University Of Massachusetts; LARGE, SCOTT - Southwest Fisheries Science Center; MORSE, RYAN - Southwest Fisheries Science Center; TOWNSEND, DAVID - University Of Maine; FRIEDRICHS, MARJORIE - Virginia Institute Of Marine Science; MELROSE, CHRISTOPHER - Southwest Fisheries Science Center

Submitted to: Journal of Shellfish Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2024
Publication Date: 8/22/2024
Citation: Fredland, K.D., Fratantoni, P., Silver, A., Brady, D.C., Gangopadhyay, A., Large, S.I., Morse, R.E., Townsend, D.W., Friedrichs, M.A., Melrose, C.D. 2024. ging source waters on the Northeast US Continental Shelf: Variation in nutrient supply and phytoplankton biomass. Journal of Shellfish Research. 281. https://doi.org/10.1016/j.csr.2024.105319.
DOI: https://doi.org/10.1016/j.csr.2024.105319

Interpretive Summary: Northeast US Continental Shelf (NES) from 1998 to 2022, focusing on the influence of various nutrient source waters. It highlights the significant variability in CHL, which is linked to changes in nutrient supply from different water masses, including Labrador Slope Water (LSW) and Warm Slope Water (WSW). The research reveals that traditional assumptions about nutrient sources may overlook the complexity of cross-shelf advection and the role of Gulf Stream warm core rings (WCRs), which have increased in frequency but contribute low nutrient loads. Additionally, the study finds that precipitation has risen over the years, affecting nutrient dynamics, although terrestrial nutrient inputs appear to be minimal. The findings suggest that understanding future CHL dynamics in the NES requires a nuanced consideration of advective nutrient sources in the Northwest Atlantic, as correlations indicate that LSW supports phytoplankton growth while WSW and WCRs do not.

Technical Abstract: The Northeast US Continental Shelf (NES) is a highly productive marine ecosystem that has experienced wide swings in phytoplankton chlorophyll concentration (CHL). To better understand this variability, we examined changes in CHL over the period 1998–2022, while also considering three indicators of the potential supply of nutrient source waters including cross-shelf advection via deep channels, transport from beyond the shelf edge via Gulf Stream warm core rings (WCR), and input from river and estuarine discharge. Traditionally, deep channel advection of water across the NES was assumed to be derived from Labrador Slope Water (LSW) and Warm Slope Water (WSW). These designations do not fully capture the range of water types contributing to cross- shelf advection. The contribution of LSW and WSW was reciprocal over time, with the presence of WSW at an increased level in recent years. There has been an increase in the number of WCRs off the NES represented by indices of ring occupancy. Precipitation increased over the study period as well, generally over the NES region and in particular in the Mid-Atlantic Bight drainage. We see evidence of the effect of increased precipitation on the NES proper through a change in the area of the ocean surface having 555 nm reflectance with sr- 1 >0.004. Using a canonical analysis, CHL correlated positively with the proportion of LSW and negatively with WSW. These correlations suggest there are aspects of the nutrient content associated with these water masses that are key to phytoplankton growth. WCR frequency negatively correlated with CHL, which was expected since the nutrient loadings of WCRs tends to be low. Finally, CHL negatively correlated with precipitation rate, which suggests terrestrial origin nutrient inputs to the NES are minor. We suggest that in order to understand future CHL dynamics in the NES, careful consideration of advective sources of nutrients in the Northwest Atlantic is necessary.