Weekly variation of seawater ecotoxicity on marine sandy beaches in relation to coliform densities

The hypothesis tested here is a direct relationship between microbiological and ecotoxicological quality on beaches. Seawater was collected weekly for one year in Northeast Brazil. The quantification of thermotolerant coliforms was obtained using the multiple tube technique. The ecotoxicological bioassay utilized the nauplii of the copepod Tisbe biminiensis. The percentage of development and survival was estimated after 72 h. Rainfall, salinity, tidal height and amplitude were analyzed. The results didn't demonstrate a relationship between the density of coliforms and lethal effects, thus rejecting the proposed hypothesis. This result suggested that sewage contamination passed through riverine plumes before arrives the beaches and coliforms died in this long way. Higher toxicity was observed during dry periods, probably due to high contaminant concentration due to lower river discharge. Other hypotheses are proposed to explain the data as higher rainfall observed in 2022 and the negative effect of population use.