Sensitivity of Aquatic Organic Matter Degradation To Changing Temperature and Nutrient Conditions in a Coastal Watershed

The degradation of organic matter (OM) within inland waters plays a pivotal role in the global carbon cycle and quantifying carbon budgets. Here, measurements of dissolved oxygen (DO) decay rates were used to infer the extent and kinetics of OM degradation under variable conditions. The goal of the investigation was to quantify how OM samples within the Waccamaw River watershed, South Carolina, respond to changes in temperature and nutrient availability as a function of their source location and lability. Samples were collected from urbanized stormwater detention ponds and undeveloped upland forested wetland drainages to provide contrasting and distinct OM sources to the river, as each possesses different degrees of OM lability. To explore the temperature sensitivity of OM degradation, samples from the summer of 2020 were incubated within three temperature regimes (20, 27.5, and 35°C) and temperature sensitivity was quantified as Q10 (relates to how a biological rate would change with 10 degrees of warming). Results indicate that temperature-driven increases in DO decay rate experienced in the more refractory, forested wetlands were more than double those experienced in the Waccamaw River and stormwater detention ponds. In summer of 2021, the potential for synergistic interactions between warming water temperatures and increasing nutrient loading from coastal development was investigated through a series of experimental enrichment experiments conducted at two temperature regimes (ambient and ambient + 5°C). Results indicated that nutrient loading produced a significant increase in DO decay rates, relative to a treatment control at ambient temperatures, approximately 42% of the time, but upon combining nutrients and warming, approximately twice as many stations (83%) yielded significant increases in DO decay rates. Additionally, the labile carbon amendments showed a significant response across the board (100%). Overall, our results suggest that naturally OM-rich systems are more susceptible to ongoing climate change, as warming temperatures allow for relatively greater OM degradation of more refractory, natural sites, even in the absence of increased urbanization. These systems will likely experience even greater DO decay rates when warming temperatures and nutrient loading increase simultaneously, which is predicted with ongoing warming and stronger, flashier storm events in the future