Impacts of Desalination Brine Discharge on Benthic Ecosystems.

Seawater reverse osmosis (SWRO) desalination facilities produce freshwater and, at the same time, discharge hypersaline brine that often includes various chemical additives such as antiscalants and coagulants. This dense brine can sink to the sea bottom and creep over the seabed, reaching up to 5 km from the discharge point. Previous reviews have discussed the effects of SWRO desalination brine on various marine ecosystems, yet little attention has been paid to the impacts on benthic habitats. This review comprehensibly discusses the effects of SWRO brine discharge on marine benthic fauna and flora. We review previous studies that indicated a suite of impacts by SWRO brine on benthic organisms, including bacteria, seagrasses, polychaetes, and corals. The effects within the discharge mixing zones range from impaired activities and morphological deformations to changes in the community composition. Recent modeling work demonstrated that brine could spread over the seabed, beyond the mixing zone, for up to several tens of kilometers and impair nutrient fluxes from the sediment to the water column. We also provide a possible perspective on brine's impact on the biogeochemical process within the mixing zone subsurface. Desalination brine can infiltrate into the sandy bottom around the discharge area due to gravity currents. Accumulation of brine and associated chemical additives, such as polyphosphonate-based antiscalants and ferric-based coagulants in the porewater, may change the redox zones and, hence, impact biogeochemical processes in sediments. With the demand for drinking water escalating worldwide, the volumes of brine discharge are predicted to triple during the current century. Future efforts should focus on the development and operation of viable technologies to minimize the volumes of brine discharged into marine environments, along with a change to environmentally friendly additives. However, the application of these technologies should be partly subsidized by governmental stakeholders to safeguard coastal ecosystems around desalination facilities.

Massive light-dependent cycling of inorganic carbon between oxygenic photosynthetic microorganisms and their surroundings.

Membrane inlet mass spectrometry indicated massive light-dependent cycling of inorganic carbon between the medium and the cells of various phytoplankton species representing the main groups of aquatic primary producers. These included diatoms, symbiotic and free living dinoflagellates, a coccolithophorid, a green alga and filamentous and single cell cyanobacteria. These organisms could maintain an ambient CO(2) concentration substantially above or below that expected at chemical equilibrium with HCO(3) (-). The coccolithophorid Emiliania huxleyi shifted from net CO(2) uptake to net CO(2) efflux with rising light intensity. Differing responses of CO(2) uptake and CO(2) fixation to changing light intensity supported the notion that these two processes are not compulsorily linked. Simultaneous measurements of CO(2) and O(2) exchange and of the fluorescence parameters in Synechococcus sp. strain PCC 7942, showed that CO(2) uptake can serve as a sensitive probe of the energy status of the photosynthetic reaction centers. However, during transitions in light intensity, changes in CO(2) uptake did not accord with those expected from fluorescence change. Quantification of the net fluxes of CO(2), HCO(3) (-) and of photosynthesis at steady-state revealed that substantial HCO(3) (-) efflux accompanied CO(2) uptake and fixation in the case of 'CO(2) users'. On the other hand, 'HCO(3) (-) users' were characterized by a rate of net CO(2) uptake below that of CO(2) fixation. The results support the notion that entities associated with the CCM function not only in raising the CO(2) concentration at the site of Rubisco; they may also serve as a means of diminishing photodynamic damage by dissipating excess light energy.

A short-term in situ CO2 enrichment experiment on Heron Island (GBR).

Ocean acidification poses multiple challenges for coral reefs on molecular to ecological scales, yet previous experimental studies of the impact of projected CO2 concentrations have mostly been done in aquarium systems with corals removed from their natural ecosystem and placed under artificial light and seawater conditions. The Coral-Proto Free Ocean Carbon Enrichment System (CP-FOCE) uses a network of sensors to monitor conditions within each flume and maintain experimental pH as an offset from environmental pH using feedback control on the injection of low pH seawater. Carbonate chemistry conditions maintained in the -0.06 and -0.22 pH offset treatments were significantly different than environmental conditions. The results from this short-term experiment suggest that the CP-FOCE is an important new experimental system to study in situ impacts of ocean acidification on coral reef ecosystems.