文摘

In estuaries, climate and nutrients are the major drivers of primary productivity, since light availability, water movements, nutrient supplies, and physical stability determine the ability of primary producers to photosynthesize, grow, and survive. Therefore, in nutrient-rich estuaries, climate is expected to be the driving force of primary productivity. As global climate changes may increase the probability of some local ordinary weather events reaching extreme levels, it becomes pertinent to understand whether climate influences estuarine primary productivity patterns over the years. By means of long-term time series of phytoplankton, seagrass (Zostera noltii), and macroalgae (Gracilaria gracilis and Ulva spp.) production measurements, the influence of local weather events upon primary productivity was investigated in a temperate nutrient-rich estuary. The hypothesis proposed that estuarine primary productivity would show temporal fluctuations linked to climate-driven events and to the mitigation measures used to combat local anthropogenic pressures. Results revealed that, in the Mondego estuary, the most primary productivity is carried out by phytoplankton and that Z. noltii leaves and G. gracilis contribute equally to the daily productivity of the estuary, followed by Z. noltii belowground and finally by Ulva spp. Phytoplankton productivity seems to reach its highest rates in dry years, whereas a considerable decrease occurred during rainy periods. Regarding Z. noltii leaves, the highest productivity rates were attained in cold years. Moreover, the results do not support any relation between macroalgae productivity and climate events, but it would appear that the absence of local anthropogenic pressures increased its production rates. In what concerns primary productivity in temperate regions, small estuaries seem to have the ability to recover from singular intense weather events occurring over the years. Since recent climate change projections point out that drought periods are expected to increase and precipitation is expected to decrease, it was possible to infer that phytoplankton, in temperate estuaries, is likely to play an important role in the fixation of carbon dioxide acting as a sink of carbon, which, in turn, may increase coastal productivity. This knowledge could contribute to reduce uncertainty in future climate change projections and to enhance management strategies at a regional level, which have important environmental and socioeconomic repercussions.