Freshwater inflow and responses from estuaries across a climatic gradient: An assessment of northwestern Gulf of Mexico estuaries based on stable isotopes

Abstract

Estuaries exist across a large climatic gradient in the northwestern Gulf of Mexico, capturing a range of hydrologic conditions and estuarine functioning. We examined freshwater inflow, salinity, and stable isotope compositions (δ13C, δ15N) of oysters, suspended particulate organic matter (SPOM), and surface sediment organic matter (SSOM) from five estuaries across the hydrologic gradient. All five estuaries experienced large decreases in freshwater inflow over the last 40 yr, with three estuaries being subject to freshwater inflow reductions of more than 85%. Generally, these freshwater inflow decreases were associated with estuarine salinity increases. Across the spatial gradient, average salinity generally increased from northeast to southwest estuaries. SPOM in the northeastern, lower salinity estuaries generally contained more continental organic matter and was of higher quality (i.e., lower C/chlorophyll a ratio), as compared to southwestern, higher salinity estuaries. Similarly, both SSOM and oyster δ13C values were positively correlated with salinity, further highlighting that food webs in lower salinity estuaries are more greatly influenced by continental organic matter than those in higher salinity estuaries. A decrease in the connectivity between continental and coastal habitats may have broad consequences for flows of organic matter, and estuarine function and health. Conducting studies across large‐scale hydrologic gradients can provide a useful approach to informing and predicting shifts in estuarine functioning.

Runoff from upstream changes the structure and energy flow of food web in estuary

“…Due to the influence of the spread of freshwater and the differing topography, environmental conditions showed differences between the northern and southern shores (Table A1). The variations in flow regimes and environmental characteristics contributed to the difference of energy flow dynamics in the food web at YRE (Maceda-Veiga et al, 2018;Liu et al, 2020;Marshall et al, 2021).…”

“…In terms of time, changes in flow regime such as magnitude of discharge, seasonal subside of terrestrial material can affect the proportion of available autochthonous/allochthonous resources for consumers (Olin et al, 2013). Higher freshwater inflow will enhance the influence of continental organic matter on estuarine food webs (Marshall et al, 2021). Hydrological regulation contributed to temporal and spatial variations of estuarine food webs simultaneously.…”

Implications of changing trends in hydroclimatic and water quality parameters on estuarine habitats in the Gulf Coastal Plain

“…Changes to freshwater flow have implications on estuarine function, influencing productivity through nutrient and organic matter availability, and altered salinity regimes that impact habitat and species distributions along the estuarine gradient. Marshall et al (2021) examined five estuaries in the northwestern Gulf of Mexico and determined that all had decreases in freshwater inflow over the last 40 years. Changes in duration and intensity of freshwater flow regimes can result in shifts to parameters influencing key foundation species within these estuarine systems, such as seagrasses and oysters.…”