Enric Saiz

Although temperature and inorganic nutrient availability are closely linked in marine ecosystems, their combined effects on key functional traits of plankton remain poorly studied. We investigated the responses of three protists, Oxyrrhis marina (heterotrophic dinoflagellate), Karlodinium armiger (mixotrophic dinoflagellate), and Strombidium arenicola (heterotrophic ciliate), to chronic warming and acute heat stress (both +3°C), under nitrogen- or phosphorus-limited conditions. Protists were fed prey with either balanced (nutrient-replete) or imbalanced (nutrient-limited) cellular stoichiometry. When prey were nutrient-replete, physiological rates increased under both warming treatments in all species; gross-growth efficiencies remained insensitive to temperature in heterotrophs but declined in the mixotroph. Under N limitation, both warming treatments generally increased growth rates in all protists while reducing ingestion rates relative to nutrient-replete conditions; gross-growth efficiencies remained similar to or higher than those under nutrient-replete diets. In contrast, P limitation reduced growth rates in heterotrophs under both warming scenarios, particularly in S. arenicola , whereas K. armiger showed overall similar performance to nutrient-balanced conditions. Cell volume decreased under both warming treatments in nutrient-replete conditions in all the species. Cell size was greatest under P limitation in all the protists, although temperature responses varied across protists. Under P limitation, C:P and N:P ratios significantly increased in all the species; however, the overall effects of temperature and nutrient stress on stoichiometric ratios differed among protists. Our results suggest that, among the species examined, the two dinoflagellates may be more tolerant of combined thermal and nutrient stress than the ciliate S. arenicola . This greater tolerance may reflect their weaker elemental regulation and, in the case of K. armiger , its mixotrophic capacity, particularly under P limitation and acute heat stress.

The impact of climate change stressors on marine copepods, key organisms at the base of ocean food webs, remains understudied. This study examined how warming affects their functional and numerical responses, critical life history traits linked to fitness. We conducted laboratory experiments on the calanoid species Paracartia grani , exposing it to a +6°C increase at two temporal scales, short term (4 d) and long term (21 generations; ca. 1 yr), and compared with the control at 19°C. We observed distinct patterns between warming treatments. Short‐term warming increased intake rates and satiation levels, causing a mismatch between the functional and the numerical responses. Additionally, egg size and phosphorus body content declined, collectively resulting in reduced reproductive efficiency at high food concentrations. Long‐term warming shifted satiation to higher food concentrations, possibly due to decreased foraging capacity (i.e., maximum clearance rate) in the smaller, warm‐reared copepods; in turn, maximum feeding rates indicated physiological compensation, and reproductive efficiency showed no consistent pattern. High food availability altered the copepod elemental content, which appeared to be linked to reproductive effort. Short‐term warming raised C : P and N : P ratios, while C : N ratios remained stable across all treatments. Our findings reveal how warming and resource availability interact to shape copepod performance and elemental fluxes, and highlight the importance of incorporating time‐scale dependent thermal effects into ecosystem models for better forecasting in a context of climate change.

We investigated the thermal tolerance and physiological responses of the heterotrophic dinoflagellate Oxyrrhis marina reared for more than 34 months at 16, 19 or 22°C, using Rhodomonas salina as prey. First, we constructed the thermal performance curves for the growth rate of the three thermal lines of O. marina and found that the maximum growth rates and the optimum temperature increased with higher rearing temperature. Next, we examined the functional responses at each rearing temperature and under acute heat stress (+3°C). Maximum ingestion rates (in cells and carbon) showed limited temperature sensitivity, regardless of chronic or acute heat exposure. Under heat stress, the 16 and 19°C lines saturated at lower prey abundances and showed enhanced clearance rates, whereas the 22°C line experienced negative effects. Growth rates increased with temperature, particularly under acute stress, while growth efficiencies remained stable under long-term thermal exposure and increased only when subjected to acute heat stress. Overall, these findings suggest O. marina follows a “Hotter is partially better” model, with notable differences between the 16 and 22°C lines. Our results highlight the importance of considering both gradual and abrupt temperature changes when predicting climate change impacts on marine food webs and biogeochemical processes.

Plankton models form the core of marine ecosystem simulators, with uses from regional resource and ecosystem management to climate change projections. In this Perspective, we suggest that stronger alignment of models with empirical knowledge about plankton physiology, diversity and trophic roles will improve model utility and the reliability of their outputs regarding biodiversity, ecophysiology, trophic dynamics and biogeochemistry. We recommend key steps to resolve the disconnect between empirical research and simulation models accounting for well-established plankton processes with an aim to increase the utility of such models for applied uses. A central challenge is characterizing the complexity of plankton diversity and activity in ways that are amenable to model incorporation. We argue that experts in empirical science are best placed to advise the development of next-generation models to address these challenges, and we propose a series of actions to achieve that engagement, including involvement of these experts in the design and exploitation of plankton digital twins.

The study of a species’ thermal tolerance and vital rates responses provides useful metrics to characterize its vulnerability to ocean warming. Under prolonged thermal stress, plastic and adaptive processes can adjust the physiology of organisms. Yet it is uncertain whether the species can expand their upper thermal limits to cope with rapid and extreme changes in environmental temperature. In this study, we reared the marine copepod Paracartia grani at control (19°C) and warmer conditions (25°C) for >18 generations and assessed their survival and fecundity under short-term exposure to a range of temperatures (11–34°C). After multigenerational warming, the upper tolerance to acute exposure (24 h) increased by 1–1.3°C, although this enhancement decreased to 0.3–0.8°C after longer thermal stress (7 days). Warm-reared copepods were smaller and produced significantly fewer offspring at the optimum temperature. No shift in the thermal breadth of the reproductive response was observed. Yet the fecundity rates of the warm-reared copepods in the upper thermal range were up to 21-fold higher than the control. Our results show that chronic warming improved tolerance to stress temperatures and fecundity of P. grani, therefore, enhancing its chances to persist under extreme heat events.

Under the current ocean warming scenario, multigenerational studies are essential to address possible adaptive changes in phenotypic traits of copepod populations. In this study, we exposed the calanoid copepod Paracartia grani, reared in the laboratory at 19°C, to warmer conditions (22°C and 25°C) to investigate the changes in key phenotypic traits in the 1^st, 10^th, and 11^th generations. Development rates and adult body size were inversely related to temperature in all generations. We also found a decline in copepod egg size at higher temperatures. Temperature had positive effects on the ingestion and egg production rate of females in the first generation, but the thermal response of the copepods diminished significantly in the consecutive generations. The decrease in thermal effects on feeding and egg production rates after multigenerational exposure cannot be explained only by the shrinkage in body size at warmer temperatures, but also involves the action of physiological compensation. These adaptive processes did not appear to have a significant cost on other traits, such as egg hatching success, gross growth efficiency, and sex ratio. Our findings have implications for the prediction of ocean warming effects on copepod activity rates and highlight the importance of physiological adaptation processes after multigenerational exposure.

With deep sadness we say goodbye to our friend and colleague Miquel Alcaraz, who passed away at age 76 on 30 April 2022. Miquel was a person of great knowledge, very lively and humane, but amongst everything, he was an excellent scientist and a passionate of nature, which he knew how to beautifully capture graphically in drawings and paintings. He graduated in Biology from the University of Barcelona in 1969 and from the early stages showed a great interest in marine ecology under the inspiration of Professor Ramon Margalef, who later would become his PhD advisor. He carried out his PhD research at the Instituto de Investigaciones Pesqueras in Vigo (Spain), where he studied the spatial and temporal segregation of congeneric species of the genus Acartia in the Ría of Vigo. He always had warm memories of the times there, in company of his wife and many friends. He frequently...

We studied the phenotypic response to temperature of the marine copepod Paracartia grani at the organismal and cellular levels. First, the acute (2 days) survival, feeding and reproductive performances at 6–35°C were determined. Survival was very high up to ca. 30°C and then dropped, whereas feeding and fecundity peaked at 23–27°C. An acclimation response developed after longer exposures (7 days), resulting in a decline of the biological rate processes. As a consequence, Q₁₀ coefficients dropped from 2.6 to 1.6, and from 2.7 to 1.7 for ingestion and egg production, respectively. Due to the similarity in feeding and egg production thermal responses, gross-growth efficiencies did not vary with temperature. Respiration rates were less sensitive (lower Q₁₀) and showed an opposite pattern, probably influenced by starvation during the incubations. The acclimation response observed in the organismal rate processes was accompanied by changes in body stoichiometry and in the antioxidant defense and cell-repair mechanisms. Predictions of direct effects of temperature on copepod performance should consider the reduction of Q₁₀ coefficients due to the acclimation response. Copepod population dynamic models often use high Q₁₀ values and may overestimate thermal effects.