Exploring coexistence mechanisms in a three-species assemblage.

Interactions among species are essential in shaping ecological communities, although it is not always clear under what conditions they can persist when the number of species involved is higher than two. Here we describe a three-species assemblage involving the seagrass Cymodocea nodosa, the pen shell Pinna nobilis and the herbivore sea urchin Paracentrotus lividus, and we explore the mechanisms allowing its persistence through field observations and manipulative experiments. The abundance of pen shells was higher in seagrass beds than in bare sand, suggesting a recruitment facilitation. The presence of sea urchins, almost exclusively attached or around pen shells, indicated habitat facilitation for sea urchins, which overgrazed the meadow around the pen shells forming seagrass-free halos. Our results suggest that this system persists thanks to: (i) the behavioral reluctance of sea urchins to move far from pen shells, making their impact on seagrass strictly local, (ii) the sparse distribution of pen shells and (iii) the plant's resistance mechanisms to herbivory. Unpacking these mechanisms allows a better understanding of how ecological communities are assembled.

Warming intensifies the interaction between the temperate seagrass Posidonia oceanica and its dominant fish herbivore Sarpa salpa.

Apart from directly influencing individual life histories of species, climate change is altering key biotic interactions as well, causing community processes to unravel. With rising temperatures, disruptions to producer-consumer relationships can have major knock-on effects, particularly when the producer is a habitat-forming species. We studied how sea surface temperature (SST) modifies multiple pathways influencing the interaction between the foundational seagrass species, Posidonia oceanica, and its main consumer, the fish Sarpa salpa in the Mediterranean Sea. We used a combination of a field-based temperature gradient approaches and experimental manipulations to assess the effect of temperature on seagrass performance (growth) and fish early life history (larval development) as well as on the interaction itself (seagrass palatability and fish foraging activity). Within the range of temperatures assessed, S. salpa larvae grew slightly faster at warmer conditions but maintained their settlement size, resulting in a relatively small reduction in pelagic larval duration (PLD) and potentially reducing dispersion. Under warmer conditions (>24 °C), P. oceanica reduced its growth rate considerably and seemed to display fewer deterring mechanisms as indicated by a disproportionate consumption in choice experiments. However, our field-based observations along the temperature gradient showed no change in fish foraging time, or in other aspects of feeding behaviour. As oceans warm, our results indicate that, while S. salpa may show little change in early life history, its preference towards P. oceanica might increase, which, together with reduced seagrass growth, could considerably intensify the strength of herbivory. It is unclear if P. oceanica meadows can sustain such an intensification, but it will clearly add to the raft of pressures this threatened ecosystem already faces from global and local environmental change.

Recovery of a fast-growing seagrass from small-scale mechanical disturbances: Effects of intensity, size and seasonal timing.

We studied the recovery of the fast-growing seagrass Cymodocea nodosa from disturbances of different intensities (shoots removal or the entire plant), plot sizes (from 0.04 to 1 m2) and in different seasons (spring and autumn) in a shallow coastal bay. We monitored recovery over 27 months and measured plant traits at the end. Shoot density and canopy height recovered faster (1 month) when only shoots were removed compared to when the entire plant was removed (10-25 months). Small areas took longer to recover than large ones, probably due to limited light availability or the accumulation of detritus. Plants disturbed in autumn took 9 months longer to recover than those disturbed in spring. After the 27-month, all plant traits were similar to those of control plots, except rhizome biomass, which was lower. Our results suggest that mechanical disturbances might exert a negative effect on the long-term resilience of seagrasses.

The negative effects of short-term extreme thermal events on the seagrass Posidonia oceanica are exacerbated by ammonium additions.

Global warming is increasingly affecting our biosphere. However, in addition to global warming, a panoply of local stressors caused by human activities is having a profound impact on our environment. The risk that these local stressors could modify the response of organisms to global warming has attracted interest and fostered research on their combined effect, especially with a view to identifying potential synergies. In coastal areas, where human activities are heavily concentrated, this scenario is particularly worrying, especially for foundation species such as seagrasses. In this study we explore these potential interactions in the seagrass Posidonia oceanica. This species is endemic to the Mediterranean Sea. It is well known that the Mediterranean is already experiencing the effects of global warming, especially in the form of heat waves, whose frequency and intensity are expected to increase in the coming decades. Moreover, this species is especially sensitive to stress and plays a key role as a foundation species. The aim of this work is thus to evaluate plant responses (in terms of photosynthetic efficiency and growth) to the combined effects of short-term temperature increases and ammonium additions.To achieve this, we conducted a mesocosm experiment in which plants were exposed to three thermal treatments (20°C, 30°C and 35°C) and three ammonium concentrations (ambient, 30 µM and 120 µM) in a full factorial experiment. We assessed plant performance by measuring chlorophyll fluorescence variables (maximum quantum yield (Fv/Fm), effective quantum yield of photosystem II (ΔF/Fm'), maximum electron transport rate (ETRmax) and non-photochemical quenching (NPQ)), shoot growth rate and leaf necrosis incidence. At ambient ammonium concentrations, P. oceanica tolerates short-term temperature increases up to 30°C. However, at 35°C, the plant loses functionality as indicated by a decrease in photosynthetic performance, an inhibition of plant growth and an increase of the necrosis incidence in leaves. On the other hand, ammonium additions at control temperatures showed only a minor effect on seagrass performance. However, the combined effects of warming and ammonium were much worse than those of each stressor in isolation, given that photosynthetic parameters and, above all, leaf growth were affected. This serves as a warning that the impact of global warming could be even worse than expected (based on temperature-only approaches) in environments that are already subject to eutrophication, especially in persistent seagrass species living in oligotrophic environments.

Interactive effects of global warming and eutrophication on a fast-growing Mediterranean seagrass.

Coastal ecosystems, such as seagrasses, are subjected to local (e.g. eutrophication) and global (e.g. warming) stressors. While the separate effects of warming and eutrophication on seagrasses are relatively well known, their joint effects remain largely unstudied. In order to fill this gap, and using Cymodocea nodosa as a model species, we assessed the joint effects of warming (three temperatures, 20 °C, 30 °C and 35 °C) with two potential outcomes of eutrophication: (i) increase in nutrients concentration in the water column (30 and 300 µM), and (ii) organic enrichment in the sediment). Our results confirm that temperature in isolation clearly affects plant performance; while plants exposed to 30 °C performed better than control plants, plants exposed to 35 °C showed clear symptoms of deterioration (e.g. decline of photosynthetic capacity, increase of incidence of necrotic tissue). Plants were unaffected by high ammonium concentrations; however, organic enrichment of sediment had deleterious effects on plant function (photosynthesis, growth, demographic balance). Interestingly, these negative effects were exacerbated by increased temperature. Our findings indicate that in addition to the possibility of the persistence of C. nodosa being directly jeopardized by temperature increase, the joint effects of warming and eutrophication may further curtail its survival. This should be taken into consideration in both predictions of climate change consequences and in local planning.

Investigating cellular stress response to heat stress in the seagrass Posidonia oceanica in a global change scenario.

Posidonia oceanica meadows are facing global threats mainly due to episodic heat waves. In a mesocosm experiment, we aimed at disentangling the molecular response of P. oceanica under increasing temperature (20 °C-32 °C). The experiment was carried out in spring, when heat waves can potentially occur and plants are putatively more sensitive to heat stress, since they are deprived in carbohydrates reserves after the cold winter months. We aimed to identify the activation of different phases of the cellular stress response (CSR) reaction and the responsive genes activated or repressed in heated plants. A molecular traffic light was proposed as a response model including green (protein folding and membrane protection), yellow (ubiquitination and proteolysis) and red (DNA repair and apoptosis) categories. Additionally, we estimated phenological trait variations to complement the information obtained from the molecular proxies of stress. Despite reduced leaf growth rate, heated plants did not exhibit signs of irreversible damage, probably underlying species pre-adaptation to warm and fluctuating regimes. Gene expression analyses revealed that molecular chaperoning, DNA repair and apoptosis inhibition processes related genes were the ones that mostly responded to high thermal stress and will be target of further investigation and in situ proofing for assessing their use as indicators of P. oceanica performance under sub-lethal heat stress.