Phytoplankton responses to marine climate change – an introduction

Phytoplankton are one of the key players in the ocean and contribute approximately 50% to global primary production. They serve as the basis for marine food webs, drive chemical composition of the global atmosphere and thereby climate. Seasonal environmental changes and nutrient availability naturally influence phytoplankton species composition. Since the industrial era, anthropogenic climatic influences have increased noticeably – also within the ocean. Our changing climate, however, affects the composition of phytoplankton species composition on a long-term basis and requires the organisms to adapt to this changing environment, influencing micronutrient bioavailability and other biogeochemical parameters. At the same time, phytoplankton themselves can influence the climate with their responses to environmental changes. Due to its key role, phytoplankton has been of interest in marine sciences for quite some time and there are several methodical approaches implemented in oceanographic sciences. There are ongoing attempts to improve predictions and to close gaps in the understanding of this sensitive ecological system and its responses

Carbon and nitrogen cycling in a shallow productive sub-tropical coastal embayment (western Moreton Bay, Australia)

Climatic variables, water quality, benthic fluxes, sediment properties, and infauna were measured six times over an annual cycle in a shallow sub-tropical embayment to characterize carbon and nutrient cycling, and elucidate the role of pelagic–benthic coupling. Organic carbon (OC) inputs to the bay are dominated by phytoplankton (mean 74%), followed by catchment inputs (15%), and benthic microalgae (BMA; 9%). The importance of catchment inputs was highly variable and dependent on antecedent rainfall, with significant storage of allochthonous OC in sediments following high flow events and remineralization of this material supporting productivity during the subsequent period. Outputs were dominated by benthic mineralization (mean 59% of total inputs), followed by pelagic mineralization (16%), burial (1%), and assimilation in macrofaunal biomass (2%). The net ecosystem metabolism (NEM = production minus respiration) varied between −4 and 33% (mean 9%) of total primary production, whereas the productivity/respiration (p/r) ranged between 0.96 and 1.5 (mean 1.13). Up to 100% of the NEM is potentially removed via the demersal detritivore pathway. Dissolved inorganic nitrogen (DIN) inputs from the catchment contributed less than 1% of the total phytoplankton demand, implicating internal DIN recycling (pelagic 23% and benthic 19%) and potentially benthic dissolved organic nitrogen (DON) fluxes (27%) or N fixation (up to 47%) as important processes sustaining productivity. Although phytoplankton dominated OC inputs in this system, BMA exerted strong seasonal controls over benthic DIN fluxes, limiting pelagic productivity when mixing/photic depth approached 1.3. The results of this study suggest low DIN:TOC and net autotrophic NEM may be a significant feature of shallow sub-tropical systems where the mixing/photic depth is consistently less than 4