Elevated pCO₂ and warming may promote algal growth and toxin production, and thereby possibly support the proliferation and toxicity of harmful algal blooms (HABs). Here, we tested whether empirical data support this hypothesis using a meta‐analytic approach and investigated the responses of growth rate and toxin content or toxicity of numerous marine and estuarine HAB species to elevated pCO₂ and warming. Most of the available data on HAB responses towards the two tested climate change variables concern dinoflagellates, as many members of this phytoplankton group are known to cause HAB outbreaks. Toxin content and toxicity did not reveal a consistent response towards both tested climate change variables, while growth rate increased consistently with elevated pCO₂. Warming also led to higher growth rates, but only for species isolated at higher latitudes. The observed gradient in temperature growth responses shows the potential for enhanced development of HABs at higher latitudes. Increases in growth rates with more CO₂ may present an additional competitive advantage for HAB species, particularly as CO₂ was not shown to enhance growth rate of other non‐HAB phytoplankton species. However, this may also be related to the difference in representation of dinoflagellate and diatom species in the respective HAB and non‐HAB phytoplankton groups. Since the proliferation of HAB species may strongly depend on their growth rates, our results warn for a greater potential of dinoflagellate HAB development in future coastal waters, particularly in temperate regions.

Intraspecific trait diversity can promote the success of a species, as complementarity of functional traits within populations may enhance its competitive success and facilitates resilience to changing environmental conditions. Here, we experimentally determined the variation and relationships between traits in 15 strains of the toxic dinoflagellate Alexandrium ostenfeldii derived from two populations. Measured traits included growth rate, cell size, elemental composition, nitrogen uptake kinetics, toxin production and allelochemical potency. Our results demonstrate substantial variation in all analysed traits both within and across populations, particularly in nitrogen affinity, which was even comparable to interspecific variation across phytoplankton species. We found distinct trade‐offs between maximum nitrogen uptake rate and affinity, and between defensive and competitive traits. Furthermore, we identified differences in trait variation between the genetically similar populations. The observed high trait variation may facilitate development and resilience of harmful algal blooms under dynamic environmental conditions.

Eutrophication has played a major role in the worldwide increase of harmful algal blooms (HABs). Higher input of key nutrients, such as nitrogen (N) and phosphorus (P), can stimulate the growth of harmful algal species in freshwater, estuarine, and coastal marine ecosystems. Some HAB-forming taxa, particularly several cyanobacteria and dinoflagellate species, are harmful through the production of N-rich toxins that have detrimental effects on the environment and human health. Here, we test how changes in nutrient availability affect N-rich toxin synthesis in cyanobacteria and dinoflagellates using a meta-analysis approach. Overall, N-rich toxin content showed an increase with P limitation, while it tended to decrease with N limitation, but we also observed substantial variation in responses both within and across genera and toxin groups. For instance, in response to N limitation, microcystin content varied from a 297% decrease up to a 273% increase, and paralytic shellfish poisoning (PSP) toxin content varied from a 204% decrease to an 82% increase. Cylindrospermopsin, produced by N₂-fixing cyanobacteria, showed no clear direction in response to nutrient limitation, and cellular contents of this compound may thus vary independently of nutrient fluctuations. Our results confirm earlier reported stoichiometric regulation of N-rich phytoplankton toxins, showing increased toxin content with an increase in cellular N:P ratios, and vice versa. Thus, changes in N-rich toxin content largely follow the changes in relative cellular N content. Consequently, although nutrient limitation may limit bloom biomass and thereby bloom toxicity, our results warn that P limitation can cause accumulation of cellular toxins and thus lead to unexpected increases in bloom toxicity.

Global climate change involves an increase in oceanic CO2 concentrations as well as\ud thermal stratification of the water column, thereby reducing nutrient supply from deep to surface\ud waters. Changes in inorganic carbon (C) or nitrogen (N) availability have been shown to affect\ud marine primary production, yet little is known about their interactive effects. To test for these\ud effects, we conducted continuous culture experiments under N limitation and exposed the bloomforming\ud dinoflagellate species Scrippsiella trochoidea and Alexandrium fundyense (formerly A.\ud tamarense) to CO2 partial pressures (pCO2) ranging between 250 and 1000 μatm. Ratios of particulate\ud organic carbon (POC) to organic nitrogen (PON) were elevated under N limitation, but also\ud showed a decreasing trend with increasing pCO2. PON production rates were highest and affinities\ud for dissolved inorganic N were lowest under elevated pCO2, and our data thus demonstrate a\ud CO2-dependent trade-off in N assimilation. In A. fundyense, quotas of paralytic shellfish poisoning\ud toxins were lowered under N limitation, but the offset to those obtained under N-replete conditions\ud became smaller with increasing pCO2. Consequently, cellular toxicity under N limitation\ud was highest under elevated pCO2. All in all, our observations imply reduced N stress under elevated\ud pCO2, which we attribute to a reallocation of energy from C to N assimilation as a consequence\ud of lowered costs in C acquisition. Such interactive effects of ocean acidification and nutrient\ud limitation may favor species with adjustable carbon concentrating mechanisms and have\ud consequences for their competitive success in a future ocean

Alexandrium ostenfeldii is among the most intensely studied marine planktonic dinophytes and in the last few years blooms have become a recurrent phenomenon mainly in brackish coastal waters. Since 2012, A. ostenfeldii recurs annually in the Ouwerkerkse Kreek, a Dutch brackish water creek discharging into an estuary with large stocks of mussels, oysters and cockles. The creek is characterized by highly dynamic abiotic conditions, notably salinity. Here, we investigated the impacts of salinities ranging from 3 to 34 on growth and toxin content of an A. ostenfeldii isolate from the creek. Our results demonstrate a broad salinity tolerance of the Dutch A. ostenfeldii population, with growth rates from 0.13 to 0.2 d⁻¹ over a salinity range from 6 to 34. Highest paralytic shellfish toxin and cyclic imine toxin cell quotas were observed for the lowest and highest salinities, and were associated with increases in cell size. Lytic activity was highest at the lowest salinity, and was 5-fold higher in the cell-free supernatants compared to cell extracts. Together our results demonstrate a tight coupling between salinity and A. ostenfeldii growth rate, cell size and toxin synthesis, which may have consequences for the seasonal dynamics of bloom toxicity.