TITLE:
Relations between Coastal Eutrophication and Sea Foam Formation during a Cold Front Passage. Case Study on Santos City Beaches (Brazilian Subtropical Coast)
AUTHORS:
Bruno Otero Sutti, Henrique José Rodrigues Dias, Nixon Claudio Sakazaki, Antonio Jeferson Muniz Almeida, Vitor Gonsalez Chiozzini, Glaucia Bueno Benedetti Berbel, Elisabete de Santis Braga
KEYWORDS:
Coastal Waters, Sea Foam, Nutrients, Eutrophication, Cold Front
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.13 No.12,
December
26,
2025
ABSTRACT: Sea foams are caused by algal blooms and can represent large areas in coastal waters during ocean fronts associated with meteorological changes that establish rainy weather and high wave incidence. Especially in urban areas along the Brazilian southeast coast, eutrophication has been evidenced as an enhancement factor for this phenomenon. The present study was conducted on Santos City beaches during foam presence, which showed a wide distribution from the surf zone up to urban structures above the high tide limit. Water and foam samples were collected from pluvial canals that connect to Santos Bay to investigate the origin of the foam and the relationship between this phenomenon and eutrophication levels. The eutrophication analysis was based on various species of nutrients (urea, ammonium, nitrite, nitrate, phosphate, and silicate), photo-pigments, and a trophic index. A cold front passage provided intense south winds (>10 m s−1) and high waves (>1 m) before the sampling day. In addition to this storm surge, an accumulated rainfall of ≈ 50 mm and a spring tide condition led to overwash at the floodgates of the pluvial canals. Salinity showed negative correlations with dissolved nutrients, indicating that increased freshwater outflows improved the trophic indices on both sides of the floodgates. At the foam sampling site, elevated concentrations of suspended particulate matter were associated with a high algal content, as primarily indicated by high chlorophyll a within particulate organic matter content. This occurred alongside high oxygen saturation and relatively low nutrient concentrations, suggesting the growth of opportunistic phytoplankton species. Our results suggest that eutrophication can intensify sea foam occurrence during overwash periods of the floodgates. However, phytoplankton analyses and more sampling sites need to be integrated into further studies for a better understanding of the local eutrophication influence on sea foam formation.