What if the upwelling weakens? Effects of rising temperature and nutrient depletion on coastal assemblages

Abstract

Surface temperature of the oceans has increased globally over the past decades. In coastal areas influenced by eastern boundary upwelling systems (EBUS) winds push seawater offshore and deep, cold and nutrient rich seawater rise toward the surface, partially buffering global warming. In North coast of Portugal, the NW Iberian upwelling system allows extensive kelp forests to thrive in these “boreal-like” conditions, fostering highly diverse and productive communities. However, the warming of the upper layer of the ocean may weaken this upwelling, leading to higher sea surface temperature and lower nutrient input in the coastal areas. The impacts in the structure and function of coastal ecosystems of these changes in the upwelling dynamics remain unexplored. The present study aimed to examine the combined effects of elevated temperature and nutrient depletion on semi-naturally structured assemblages. The eco-physiological responses explored included growth, chlorophyll fluorescence and metabolic rates at individual and assemblage’s levels. Our results showed synergistic effects of the combination of elevated temperature and lack of nutrients on the large canopy forming species (i.e. kelp), those effects drove community responses with significant losses in productivity levels. Smaller assemblage components like (sub-canopy and turf specie) were affected in an additive way (Subcanopy species Chondus crispus) or only affected by temperature (Turf). Our results suggest that under weakening upwelling scenarios, the ability of the macroalgal assemblages to maintain high productivity rates could be seriously affected and predict a shift in community composition with the loss of marine forests.