Changes in microbial communities in response to submarine groundwater input

The effects of submarine groundwater on the native plankton populations of a coastal area were examined through a series of in situ groundwater addition experiments carried out during the summer in a Mediterranean embayment. Different percentages (4, 8, 10 and 12%) of groundwater extracted from 2 intertidal coastal localities subjected to different land uses were added to picoplankton-dominated natural populations. The responses of the phytoplankton and bacterioplankton biomass were analyzed. The biomass of the phytoplankton community increased by as much as 96% above the mean initial value in ammonium-enriched groundwater and by a maximum of 400% in groundwater enriched in nitrate. Groundwater additions were followed by an enhancement in the biomass of all major autotrophic groups, with the most notable response in picophytoplankton, whereas bacterioplankton abundance increased only slightly. The abundance of diatoms was initially low although their growth rates increased faster than those of dinoflagellates, thus shifting the community composition towards a higher relative microphytoplankton proportion. An initial dinoflagellate community dominated by small naked dinoflagellates shifted to one characterized by a high abundance of Prorocentrum minimum. Our results demonstrate that, even in areas with low anthropogenic activity, groundwater discharges to the coast can effectively stimulate autotrophic plankton growth, thereby producing shifts in the microbial food-web structure of coastal waters. This, in turn, increases the possibility of outbreaks of opportunistic species, which can eventually result in harmful algal bloom episodes.