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

## TL;DR

This study examines how nutrients and organic matter from human activities affect coastal sediments and water quality in a semi-enclosed bay.

## Contribution

The study identifies key factors influencing nutrient distribution in sediments and porewater under combined urban and aquaculture pressures.

## Key findings

- Sediment phosphorus levels were higher near sewage and mussel farming areas.
- Porewater showed high dissolved nitrogen and phosphorus, suggesting potential release to the water column.
- Cluster analysis revealed two distinct sediment groups based on organic matter accumulation patterns.

## Abstract

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.

The online version contains supplementary material available at 10.1007/s10661-026-15065-y.

Nutrient behavior in the porewater controls water quality.Eh determines nutrient and organic matter retention/release from sediments.Breaking waves modify physicochemical conditions and affect nutrients on the shoreline.Nutrient inputs from drainages are relevant, but do not always determine enrichment.Mussel farms reduce hydrodynamics, leading to nutrient accumulation.

Nutrient behavior in the porewater controls water quality.

Eh determines nutrient and organic matter retention/release from sediments.

Breaking waves modify physicochemical conditions and affect nutrients on the shoreline.

Nutrient inputs from drainages are relevant, but do not always determine enrichment.

Mussel farms reduce hydrodynamics, leading to nutrient accumulation.

The online version contains supplementary material available at 10.1007/s10661-026-15065-y.