The Mediterranean benthic herbivores show diverse responses to extreme storm disturbances

Nutrient conditions determine the strength of herbivore-mediated stabilizing feedbacks in barrens

Abiotic environmental conditions can significantly influence the way species interact. In particular, plant-herbivore interactions can be substantially dependent on temperature and nutrients. The overall product of these relationships is critical for the fate and stability of vegetated ecosystems like marine forests. The last few decades have seen a rapid spread of barrens on temperate rocky reefs mainly as a result of overgrazing. The ecological feedbacks that characterize the barren state involve a different set of interactions than those occurring in vegetated habitats. Reversing these trends requires a proper understanding of the novel feedbacks and the conditions under which they operate. Here, we explored the role of a secondary herbivore in reinforcing the stability of barrens formed by sea urchin overgrazing under different nutrient conditions. Combining comparative and experimental studies in two Mediterranean regions characterized by contrasting nutrient conditions, we assessed: (i) if the creation of barren areas enhances limpet abundance, (ii) the size-specific grazing impact by limpets, and (iii) the ability of limpets alone to maintain barrens. Our results show that urchin overgrazing enhanced limpet abundance. The effects of limpet grazing varied with nutrient conditions, being up to five times more intense under oligotrophic conditions. Limpets were able to maintain barrens in the absence of sea urchins only under low-nutrient conditions, enhancing the stability of the depauperate state. Overall, our study suggests a greater vulnerability of subtidal forests in oligotrophic regions of the Mediterranean and demonstrates the importance of environment conditions in regulating feedbacks mediated by plant-herbivore interactions.

Sublethal effects of a rapidly spreading native alga on a key herbivore

Multiple anthropogenic stressors are causing a global decline in foundation species, including macrophytes, often resulting in the expansion of functionally different, more stressor-tolerant macrophytes. Previously subdominant species may experience further positive demographic feedback if they are exposed to weaker plant-herbivore interactions, possibly via decreased palatability or being structurally different from the species they are replacing. However, the consequences of the spread of opportunistic macrophytes for the local distribution and life history of herbivores are unknown.The green alga, Caulerpa filiformis, previously a subdominant macrophyte on low intertidal-shallow subtidal rock shores, is becoming locally more abundant and has spread into warmer waters across the coast of New South Wales, Australia.In this study, we measured (a) the distribution and abundance of a key consumer, the sea urchin Heliocidaris erythrogramma, across a seascape at sites where C. filiformis has become dominant, (b) performed behavioral field experiments to test the role of habitat selection in determining the local distribution of H. erythrogramma, and (c) consumer experiments to test differential palatability between previously dominant higher quality species like Ecklonia radiata and Sargassum sp. and C. filiformis and the physiological consequences of consuming it.At all sites, urchin densities were positively correlated with distance away from C. filiformis beds, and they actively moved away from beds. Feeding experiments showed that, while urchins consumed C. filiformis, sometimes in equal amounts to higher quality algae, there were strong sublethal consequences associated with C. filiformis consumption, mainly on reproductive potential (gonad size). Specifically, the gonad size of urchins that fed on C. filiformis was equivalent to that in starved urchins. There was also a tendency for urchin mortality to be greater when fed C. filiformis.Overall, strong negative effects on herbivore life-history traits and potentially their survivorship may establish further positive feedback on C. filiformis abundance that contributes to its spread and may mediate shifts from top-down to bottom-up control at locations where C. filiformis has become dominant.

Fishery management in a marine protected area with compliance gaps: Socio-economic and biological insights as a first step on the path of sustainability

Overfishing is one of the main impacts on the marine environment and multiple-use Marine Protected Areas (MPAs) could be a useful tool to conserve biodiversity and promote sustainable resource exploitation. However, ensuring a high level of protection on the ground is a difficult task. This work contributes to the analysis of the causes at the root of MPAs' ineffectiveness by examining the management of Paracentrotus lividus fishery in an Italian MPA, employing a multidisciplinary approach built on biological and socio-economic competences. This sea urchin species has a determinant ecological role in structuring infralittoral benthic assemblages and is the most exploited echinoid in Europe. From 2010 to 2018, underwater sampling was conducted over 39 monitoring sites to define P. lividus spatial and temporal trends. Declared catches and semi-structured interviews with local stakeholders were used to define the socio-economical context, underline existing conflicts among them, as well as to trace the historical evolution of sea urchin fishery. The results show that the management of sea urchin fishery is not sustainable, primarily because of the stakeholders' non-compliance with the rules. P. lividus stock is progressively declining (-73% in 9 years), showing no difference between MPA (0.5 ± 0.15 ind./m²) and control sites (0.3 ± 0.04 ind./m²). Moreover, fishermen dominate the social arena while scientists, civil society and local press have little relevance. Additionally, the untruthfulness of catch declarations was proved, the IUU fishery is relevant and the black market is hiding the actual economic value. This work offers management solutions that may be useful in other areas that show similar compliance issues.

Sensitivity of Storm-Induced Hazards in a Highly Curvilinear Coastline to Changing Storm Directions. The Tordera Delta Case (NW Mediterranean)

Extreme coastal storms, especially when incident in areas with densely urbanized coastlines, are one of the most damaging forms of natural disasters. The main hazards originating from coastal storms are inundation and erosion, and their magnitude and extent needs to be accurately assessed for effective management of coastal risk. The use of state-of-art morphodynamic process-based models is becoming standard, with most being applied to straight coastlines with gentle slopes. In this study, the XBeach model is used to assess the coastal response of a curvilinear sensitive deltaic coast with coarse sediment and steep slopes (intermediate-reflective conditions). The tested hypothesis is that changes in wave direction may cause large variations in the magnitude of storm-induced hazards. The model is tested against field data available for the Sant Esteve Storm (December 2008), obtaining an overall BSS (Brier Skill Score) score on the emerged morphological response of 0.68. Later, the 2008 event is used as baseline scenario to create synthetic events covering the range from NE to S. The obtained results show that storm-induced hazards along a highly curvilinear coast are very sensitive to changes in wave direction. Therefore, even under climate scenarios of relatively steady storminess, a potential shift in wave direction may significantly change hazard conditions and thus, need to be accounted for in robust damage risk assessments.

No-take marine reserves control the recovery of sea urchin populations after mass mortality events

Understanding how no-take zones (NTZs) shape the population dynamics of key herbivores is crucial for the conservation and management of temperate benthic communities. Here, we examine the recovery patterns of sea urchin populations following a high-intensity storm under contrasting protection regimes in the NW Mediterranean Sea. We found significant differences in the recovery trends of Paracentrotus lividus abundance and biomass in the five years following the storm. The P. lividus populations outside the NTZ recovered faster than the populations inside the NTZ, revealing that predation was the main factor controlling the sea urchin populations inside the NTZ during the study period. Arbacia lixula reached the highest abundance and biomass values ever observed outside the NTZ in 2016. Our findings reveal that predation can control the establishment of new sea urchin populations and emphasize top-down control in NTZs, confirming the important role of fully protected areas in the structure of benthic communities.

Thermal stratification drives movement of a coastal apex predator

A characterization of the thermal ecology of fishes is needed to better understand changes in ecosystems and species distributions arising from global warming. The movement of wild animals during changing environmental conditions provides essential information to help predict the future thermal response of large marine predators. We used acoustic telemetry to monitor the vertical movement activity of the common dentex (Dentex dentex), a Mediterranean coastal predator, in relation to the oscillations of the seasonal thermocline during two summer periods in the Medes Islands marine reserve (NW Mediterranean Sea). During the summer stratification period, the common dentex presented a clear preference for the warm suprathermoclinal layer, and adjusted their vertical movements following the depth changes of the thermocline. The same preference was also observed during the night, when fish were less active. Due to this behaviour, we hypothesize that inter-annual thermal oscillations and the predicted lengthening of summer conditions will have a significant positive impact on the metabolic efficiency, activity levels, and population dynamics of this species, particularly in its northern limit of distribution. These changes in the dynamics of an ecosystem's keystone predator might cascade down to lower trophic levels, potentially re-defining the coastal fish communities of the future.

Ordinary and Extraordinary Movement Behaviour of Small Resident Fish within a Mediterranean Marine Protected Area

It is important to account for the movement behaviour of fishes when designing effective marine protected areas (MPAs). Fish movements occur across different spatial and temporal scales and understanding the variety of movements is essential to make correct management decisions. This study describes in detail the movement patterns of an economically and commercially important species, Diplodus sargus, within a well-enforced Mediterranean MPA. We monitored horizontal and vertical movements of 41 adult individuals using passive acoustic telemetry for up to one year. We applied novel analysis and visualization techniques to get a comprehensive view of a wide range of movements. D. sargus individuals were highly territorial, moving within small home ranges (< 1 km²), inside which they displayed repetitive diel activity patterns. Extraordinary movements beyond the ordinary home range were observed under two specific conditions. First, during stormy events D. sargus presented a sheltering behaviour, moving to more protected places to avoid the disturbance. Second, during the spawning season they made excursions to deep areas (> 50 m), where they aggregated to spawn. This study advances our understanding about the functioning of an established MPA and provides important insights into the biology and management of a small sedentary species, suggesting the relevance of rare but important fish behaviours.