Qualitative changes in macroalgal assemblages under two contrasting climatic conditions in a subtropical estuary

Abstract

In this study, we examined the qualitative changes in macroalgal assemblages during the climatic phenomena of drought and rainy season in 45 sampling stations along Estero de Urias in the Gulf of California. The lagoon has a mouth that connects it to the ocean, but there is no continuous supply of fresh water, and physico-chemical characteristics are highly influenced by the contrasting weather in this region. The ratio of green algae versus total species per sampling station was used as an indicator of change in organic enrichment. The ratio was high during both seasons, and the nutrient concentration averaged 14.8 mug at N/L and 1.0 muM dissolved-P during the drought and 12.4 mug at N/L and 1.7 muM dissolved-P during the rainy season, showing that the estuary has constant high levels of nutrients in both seasons. However, the average number of species per sampling station, and the number of stations with macroalgae were both larger during the drought season, mainly with Ulva lactuca, U. lobata and Enteromorpha clathrata. In contrast. during the rainy season the assemblages in some parts of the estuary are dominated by red seaweeds such as Gracilariopsis sjoestedtii and Caloglossa leprieurii. Moreover, in spite of the drought season (when it is not possible to observe clear assemblages), during the rainy season four main groups of stations were established by means of a classification analysis based upon the number of species that occur in the different stations. It seems that during the drought the system is environmentally more homogeneous than during the rains, when there is a slight improvement of water conditions, and different environmental zones are established.