Carbon Dynamics on the Louisiana Continental Shelf and Cross-Shelf Feeding of Hypoxia

Abstract

Large-scale hypoxia regularly develops during the summer on the Louisiana continental shelf. Traditionally, hypoxia has been linked to the vast winter and spring nutrient inputs from the Mississippi River and its distributary, the Atchafalaya River. However, recent studies indicate that much of the shelf ecosystem is heterotrophic. We used data from five late July shelfwide cruises from 2006 to 2010 to examine carbon and oxygen production and identify net autotrophic areas of phytoplankton growth on the Louisiana shelf. During these summer times of moderate river flows, shelfwide pH and particulate organic carbon (POC) consistently showed strong signals for net autotrophy in low salinity (<25) waters near the river mouths. There was substantial POC removal via grazing and sedimentation in near-river regions, with 66-85 % of POC lost from surface waters in the low and mid-salinity ranges without producing strong respiration signals in surface waters. This POC removal in nearshore environments indicates highly efficient algal retention by the shelf ecosystem. Updated carbon export calculations for local estuaries and a preliminary shelfwide carbon budget agree with older concepts that offshore hypoxia is linked strongly to nutrient loading from the Mississippi River, but a new emphasis on cross-shelf dynamics emerged in this research. Cross-shelf transects indicated that river-influenced nearshore waters <15 m deep are strong sources of net carbon production, with currents and wave-induced resuspension likely transporting this POC offshore to fuel hypoxia in adjacent mid-shelf bottom waters.

Physical and Biogeochemical Controls on pH Dynamics in the Northern Gulf of Mexico During Summer Hypoxia

“…The net autotrophic production led to corresponding increases in DO and pH in the plume regions (Figures and ). This “optimal growth region” (DeMaster et al, ; Fry et al, ) with high production rates existed in both plume systems where light and nutrient availability were both favorable for phytoplankton growth. When NH 4 + or NO 3 − is used as nitrogen source, the synthesis of organic matter can be written as (Anderson, ; Gruber & Sarmiento, ) …”

Temporal variation and stoichiometric ratios of organic matter remineralization in bottom waters of the northern Gulf of Mexico during late spring and summer

“…Nonetheless, our findings are consistent with those of Fry et al . [], who concluded that the bottom water oxygen consumption rate was comparable with riverine nutrient loading, if one assumed that hypoxic volume was ventilated and reestablished, on average, five times per year. Moreover, we calculated that the ratios of the DIC loss and that of N and P from vertical mixing were about 500/9/1, much higher than the Redfield ratios 106/16/1, and that may have caused the departure of ΔC org from Redfield ratio in the OC remineralization ratio model simulation (see details in 4.4).…”

Satellite observation of particulate organic carbon dynamics on the Louisiana continental shelf

“…However, annual mean POC and river discharge were not correlated on the inner shelf, which suggests other sources or processes regulating POC dynamics. For example, local marsh inputs may contribute 19–34% of the organic carbon on the inner shelf [ Bianchi et al ., ] and wind‐driven sediment resuspension and transport on the shallow inner shelf may also be significant [ Fry et al ., ].…”

“…Several studies have indicated that remineralization and remobilization also play important roles in controlling POC variation on the shelf [ Schlu¨nz and Schneider , ; Mckee et al ., ; Bianchi and Bauer , ]. Although substantial efforts have been made to understand POC dynamics on the LCS, previous studies had limited spatial and temporal coverage [ Trefry et al ., ; Bianchi et al ., ; Wang et al ., ; Allison et al ., ; Waterson and Canuel , ; Lehrter et al ., ; Fry et al ., ].…”