Factors determining nutrient distribution in sediments and porewater in a semi-confined coastal environment subject to anthropogenic pressure

Increasing inputs of nutrients and organic matter from urban areas accumulate in coastal sediments, which act as a storage compartment that can trigger eutrophication events, and promote organism mortality. The aim of this research was to evaluate the factors that determine the distribution of nutrients in sediments and porewater from a coastal environment. Jurujuba cove receives nutrients and organic matter inputs from diffuse sources (mainly domestic sewage) and from intensive mussel farming. Fifteen sediment samples were collected. pH and Eh were measured in situ. Sediments were centrifuged to extract porewater, where dissolved nutrients were analyzed. Total phosphorus and total organic carbon were analyzed in the bulk sediment by spectrophotometry and titration, respectively. Granulometry was analyzed with a laser granulometer, and carbonates were measured gravimetrically. Sediment phosphorus concentrations ranged between 0.2 and 3.2 mg g^-1 and were possibly related to sewage and aquaculture, with higher concentrations at the Cachoeira River mouth and close to mussel farms. The results indicated high dissolved nitrogen (2.2 ± 4.8 mg L^-1) and phosphorus (0.3 ± 0.3 mg L^-1) concentrations in porewater, indicating possible diffusive release to the water column. The Q-mode cluster analysis showed the formation of two distinct groups, determined by the characteristics of the sediments. The first group incorporates samples located near the shoreline, representing an environment with higher accumulation of organic matter (average 16.6%). The other group comprised stations in the middle of the inlet, showing slightly smaller concentrations of organic matter (average 12.0%). Nonetheless, overlapping physicochemical conditions made it difficult to clearly separate the influence of urban drainage and mussel farming, indicating that multiple stressors act simultaneously in the cove. The application of isotopic relationships (nitrogen mainly) would contribute to the depiction of sources in the research area.