Environmental assessment of three different sites in shallow environments of La Paz Bay (Gulf of California) using the TRIX index and macroalgae biomass. Part I

Coastal areas are of particular ecological and economic importance as they provide direct and indirect goods and services to society, both biological indicators and water quality assessment are of importance when monitoring these environments. Hydrographical parameters, nutrients, chlorophyll-a, and macroalgae biomass were measured, and the TRIX index was obtained, at three sites of La Paz Bay subjected to different types of anthropogenic influence: San Juan de la Costa (SJC), Casa del Marino (CM), and El Tecolote (TE), during two months of 2017 and four months of 2018. Also, macroalgae blooms at these sites were monitored by sampling the biomass of key species. Temperature showed the typical seasonal variation, from 21 °C in winter to 28 °C in summer, while salinity showed little variation (35–37 psu). Chlorophyll-a varied in parallel with changes in water temperature (0.3–2.4 μg L⁻¹), while nutrients varied depending on the season and site. The TRIX index showed significant differences between seasons, with the highest values in the warmer months (6.0) and the lowest in November (2.7); the sites studied ranged from mesotrophic to oligotrophic. Four macroalgae species accounted for over 60% of the biomass during the sampling period: Ulva ohnoi (16.1 kg), Spyridia filamentosa (6.5 kg), Acanthophora spicifera (4.2 kg), and Ulva sp. 1 (2.7 kg). Ulva, Gracilaria, and Hypnea species showed a positive significant correlation with nutrients and the TRIX index. A high macroalgae biomass may be related to a mesotrophic state, showing its value for determining the health status of coastal ecosystems and issuing proper management recommendations.