The use of antifouling booster biocides on boats and ships is of environmental concern as the leached products can affect non-target, highly susceptible phytoplanktonic organisms. Appropriate assessment of whether or not estuarine/marine phytoplankton are potentially at risk should include a combination of approaches, investigating toxic effects for both single species and natural phytoplankton communities and ideally include field verifications. This research addresses the application and comparison of methods to investigate toxic responses to four antifouling biocides Irgarol 1051®, diuron, Sea-Nine 211® and zinc pyrithione in laboratory-based experiments with single species and using natural phytoplankton assemblages. The applicability of photosynthetic parameters and pigment:chlorophyll-α ratios as biomarkers was initially employed in unialgal experiments. Variations in results for pigment:chlorophyll-a ratios following toxic exposures indicate that care must be taken when using CHEMTAX estimations and that further research into this topic is needed. Compositional changes in natural phytoplankton assemblages under toxicant exposure were evaluated using standard techniques (microscopy) and were compared to group-specific biomass estimation derived from CHEMTAX-High Performance Liquid Chromatography. This broad approach proved effective in detecting changes in the main phytoplankton groups, with prasinophytes and prymnesiophytes proving most susceptible and chlorophytes and dinoflagellates being comparatively resistant to the PSII inhibitor Irgarol 1051®. In addition, the impact of selected antifouling agents on photosynthesis and pigment chemotaxonomy was investigated for a natural phytoplankton community. Pigment signatures were determined by HPLC and growth was determined by Analytical Flow Cytometry (AFC). Primary production (estimated by 14C-uptake) was compared to photosynthetic efficiency (FV/FM) measured using Fast Repetition Rate Fluorescence (FRRF). Differences in species-specific sensitivity of the phytoplankton community were detected through pigment composition after 72 h exposures to zinc pyrithione (5 µg 1ˉ¹) and Sea-Nine 211® (10 µg lˉ¹) . The pigment zeaxanthin was proportionally increased indicating a relative increase in Cyanophyceae. This effect was corroborated by AFC. Both techniques (14C-uptake and FRRF) were in good agreement (r = 0.88) suggesting the impairment of primary production and FV/FM following exposure to the selected toxicants. Phytoplankton responses to toxicants may be influenced by fluctuations in a variety of environmental factors. The nutrient regime may alter the outcome of contamination by influencing the susceptibility of phytoplankton to a toxicant. A preliminary investigation of such effects on biocidal toxicity was conducted under controlled laboratory conditions to determine the extent of nutrient influence. Results indicated the influence of nutrient addition on the Irgarol 1051® toxic responses to cyanophytes. Besides experimental approaches, data were supplemented with a pilot survey in Plymouth coastal waters. Current concentrations of biocides together with environmental and biological data for this survey are presented and discussed as a diagnosis that provides an outline for future research.

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