Influence of major storm events on the quantity and composition of particulate organic matter in the Mission-Aransas Estuary
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Variations in the freshwater inflow regimes of estuaries, due to perturbations such as storm events, alter the source and composition of particulate organic matter (POM), which represents the base of the food web, including available energy resources for secondary production. To evaluate the impact of varying freshwater discharge on POM quantity and composition, monthly samples of surface water were collected at four long term monitoring stations from 2012-2015 in the Mission-Aransas Estuary in subtropical Texas, which is characterized by a semiarid climate, drought, and sporadic precipitation events that affect water quality of the system. We analyzed organic carbon and nitrogen concentrations, carbon and nitrogen stable isotope ratios, and pigment concentrations of the POM samples. Following a prolonged dry spell, consecutive major storm events in spring 2015 led to a dramatic freshening of the entire estuary. Large increases in particulate organic carbon (POC) concentrations and decreases in δ¹³C values of POC over several weeks following the storms suggest an increase in in-situ production at lower salinities as a result of increased inflows. These changes in bulk POM coincided with an increase in chlorophyll a concentration, further indicating a significant contribution by phytoplankton to the elevated POM. Concurrently, pigment biomarkers revealed a significant (10-fold) increase in the cyanobacterial pigment, zeaxanthin, and further 16S rRNA analysis showed that Cyanobium spp. was responsible for the observed bloom. The combination of environment conditions, including freshening, high temperature and high nutrients, likely contributed to the cyanobacteria bloom. These results show episodic rain events can substantially affect estuarine phytoplankton community composition, which impacts the available energy resources for secondary production and thus may have ecosystem-wide implications on productivity.