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Discerning the biogeochemical response of temperate estuaries to increasing temperature and precipitation through stable isotopes
Citation:
Balint, S., A. Oczkowski, M. Schwartz, M. Potter, A. Pimenta, Alana Hanson, R. McKinney, C. Oviatt, AND R. Fulweiler. Discerning the biogeochemical response of temperate estuaries to increasing temperature and precipitation through stable isotopes. Coastal & Estuarine Research Federation Conference, Portland, OR, November 12 - 16, 2023.
Impact/Purpose:
Managers are concerned about the amount of nitrogen in coastal waterbodies. In Narragansett Bay, Rhode Island, they have worked to reduce nitrogen from sewage treatment plants, successfully reducing nitrogen loads from these sources by more than 60%. As researchers continue to collect data, both from observations made in the bay and through experimental work, there is emerging evidence that other sources of nitrogen, from nitrogen fixation and precipitation, may also be important, significant sources to coastal waterbodies.
Description:
The co-occuring impacts of managed nutrient reductions and climate change have greatly complicated our understanding of nutrient dynamics in temperate estuaries. Such is the case in Narragansett Bay, RI where an estimated >60% reduction in nitrogen (N) loading over the past two decades has been undertaken in the context of increasing temperature and precipitation. Particulate organic matter collected in the Bay over 8 years has exhibited intermittent periods of anomalously low (<3‰) δ15N, suggesting substantial N sources beyond wastewater. To further explore non-point N sources and the impacts of temperature and precipitation on estuarine N cycling, we present 1) the stable isotope composition of particulate organic matter collected in temperature-controlled mesocosms with -3°C, ambient, and +3°C treatments; 2) bi-weekly particulate organic matter sampled from the upper Narragansett Bay concurrently with physical water quality parameters for 1 year, and; 3) correlation analysis and random forest modelling of monthly particulate organic matter sampled from the upper Narragansett Bay collected over 8 years. Significant differences between warm and cold treatments in the mesocosms were observed for both δ15N (p = 0.016) and δ13C (p = 0.020), suggesting that the warmer tanks were experiencing greater productivity potentially due to N fixation. A substantial (>8‰) depletion in δ15N during nutrient fertilization of the mesocosms mirrors anomalous δ15N events in Narragansett Bay, but kinetic fractionation may be contributing to the observed change in isotope composition. Particulate δ15N sampled weekly from the upper Bay exhibited significant linear relationships with salinity (p < 0.001), chlorophyll concentration (p < 0.001), and precipitation amount (p < 0.001), with stronger relationships evident in the northern portions of the Bay. Statistical interpretation of the 8-year monthly dataset shows similar trends, with significant linear relationship observed between particulate δ15N and salinity (p < 0.001), precipitation (p = 0.009), river discharge (p < 0.001), and dissolved inorganic nutrient concentration (p < 0.001). Random forest modeling further suggests that stable isotopes are strongly predicted by these factors. Taken together, these results build on a growing body of evidence that that N-fixation and precipitation-derived N (e.g., stormwater runoff, atmospheric deposition) are underestimated sources of N in this urbanized estuary that will become more important with climate change.
