Coastal marine waters are experiencing significant environmental changes (e.g. increasing water temperature) that are influencing the dynamics and increased occurrence of harmful microalgal blooms (HABs) worldwide. Harmful blooms from benthic microalgae (e.g. dinoflagellates) are annually reported in some UK waters but little research has been conducted to date regarding the impact of a global warming scenario on their abundance and toxin production Therefore, this study aimed to investigate the effect of increasing water temperature on the growth and toxin production of three toxigenic/benthic dinoflagellates isolated from the Fleet Lagoon, Dorset, UK. The harmful/epibenthic dinoflagellates Amphidinium car/erae, Prorocen/rum lima, and Coolia mono/is were isolated from the Fleet Lagoon and grown in clonal batch non axenic monocultures between 5 and 30 ºC using Guillards F/2 medium. Cultures were maintained at irradiances between 35 and 65 umol m•2 S-I and under a 12h L: 12h D photoperiod. A single strain was used of each microalgal species and samples for cell abundance, photosynthetic efficiency (Fv/Fm), inorganic nutrient concentration (nitrate, N03+N02 and phosphate, P04), chlorophy 11 a and toxin measurements were collected every few days over periods of up to 80 days. Toxins were detected and measured by LC-MS/MS from P lima samples while copepod and brine shrimp bioassays and haemolytic assays were used to estimate the potential toxicity of A. carterae and C. monotis. A. carterae cell growth rate produced a strong linear relationship (r2= 0.97, p= 0.001) with temperature, with higher growth rates (~=0.14-0.55 d') at increasing growth temperatures ranging between 5 and 25 QC. Photosynthetic efficiency (Fv/Fm values) increased with temperature from 5 to 25 QC but showed a strong decrease at 30 QC. A. carterae growth produced higher N03+N02 and P04 consumption at increasing temperatures resulting in P04 limitation at temperatures between 15-25QC. Toxin assays confirmed that A. carterae produced hemolysins causing deleterious effects on red blood cells and mortality in harpacticoid copepods. P lima cell growth rate was linearly related (r2= 0.97, p= <0.00 I) to temperature between 5 QC (u= 0.05 d-I) and 15 QC (u= 0.17 d') with similar growth rates at 15 and 25 QC. Maximum Fv/ Fm values increased at higher temperatures, but there was a lack of coincidence between the highest Fv/Fm value (at 20 QC) and highest growth rate. N03+N02 and P04 uptake ratios showed that P Lima cells removed nutrients more efficiently at increasing temperatures, causing P04 limitation at 20-30 QC. N03+N02 and P04 uptake in cultures, particularly P04 depletion during the stationary growth phase, was linked with higher concentrations of the toxins okadaic acid (OA) and dinophysistoxin 1 (DTX I). Furthermore, toxin production was influenced by increasing growth temperatures, Total OA varied between 2.0-10.99 pg cell' while Total DTXl ranged between 0.82-5.96 pg cell" in P. Lima cells, with OA/DTXI ratios between 1.7 and 2.9. P. Lima was highly toxic to Artemia salina with toxins causing a rapid rate of mortality « 24h). C. mono/is cells growth rates also increased linearly (u= 0.003 to 0.24 d"; r2= 0.69, p= <0.001) with increasing growth temperatures between 5 and 20 QC. The highest Fv/Fm value was determined in C. mono/is cells at 15 QC and higher or lower temperatures caused a reduction of Fv/ Fm values. N03+N02 and P04 were substantially removed from culture flasks at increasing temperatures between 10 and 25 QC and P04 limitation occurred at these temperatures towards the end of the stationary growth phase. Haemolytic compounds were detected in C. mono/is cells and bioassays confirmed a toxic effect of cell extracts on red blood cells. This study provides evidence that the toxigenic dinoflagellates A. carterae, C. mono/is, and P lima can tolerate and grow at a range of temperatures suggesting that increasing sea water temperatures will have a positive effect on the growth of these toxigenic/epibenthic dinoflagellates, particularly under N03+N02 and P04 replete conditions. Theses toxic dinoflagellates can therefore be considered a threat to the Fleet Lagoon which will become a particular problem as water temperatures increase.