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

The dominant seagrass herbivore Sarpa salpa shifts its shoaling and feeding strategies as they grow

The relative benefits of group foraging change as animals grow. Metabolic requirements, competitive abilities and predation risk are often allometric and influenced by group size. How individuals optimise costs and benefits as they grow can strongly influence consumption patterns. The shoaling fish Sarpa salpa is the principal herbivore of temperate Posidonia oceanica seagrass meadows. We used in-situ observations to describe how ontogeny influenced S. salpa individual feeding behaviour, shoaling behaviour and group foraging strategies, and its potential consequences to seagrass meadows. Shoaling was strongly influenced by body length: shoals were highly length-assorted and there was a clear positive relationship between body length and shoal size. Foraging strategies changed dramatically with shoal size. Small shoals foraged simultaneously and scattered over large areas. In contrast, larger shoals (made of larger individuals) employed a potentially cooperative strategy where individuals fed rotationally and focused in smaller areas for longer times (spot feeding). Thus, as individuals grew, they increased their potential impact as well, not merely because they consumed more, but because they formed larger shoals capable of considerably concentrating their grazing within the landscape. Our results indicate that ontogenetic shifts in group foraging strategies can have large ecosystem-wide consequences when the species is an important ecosystem modifier

The effect of a centenary storm on the long-lived seagrass Posidonia oceanica

We used the disturbance resulting from a once in a 100‐yr storm on the northwest Mediterranean coast to examine the extent of the disturbance, the tolerance thresholds to burial, and the medium‐term response of the long‐lived Posidonia oceanica seagrass. Sediment burial at 12 surveyed areas was particularly strong in shallow meadows, with 23% of their surfaces buried, on average, under more than 10 cm of sediment. In contrast, less than 5% of the meadow was affected at deeper locations. At three sites, we tracked short‐term mortality along a gradient of sediment burial. Survival response to burial was clearly nonlinear, with a significant threshold at 4-5 cm, beyond which shoot mortality was 100%. To track medium‐term potential recovery, we established permanent plots subject to three sediment burial levels (0-5, 5-10, and > 10 cm burial) in four meadows. Where the initial shoot mortality was 100%, we recorded no shoot recovery over the 4‐yr period. In the remaining plots, where some shoots remained alive, we detected either further mortality or shoot recovery of 7% per year on average. Extreme storm events can result in sudden catastrophic losses of seagrass cover in shallow P. oceanica meadows. In the long term and due to the long return time of such storms, the species may still be able to recover despite its low recovery potential. However, added anthropogenic stressors, including climate change, may seriously test the ability of long‐lived shallow seagrass ecosystems to resist high‐intensity natural disturbances and may be critical for its persistence

Pseudovivipary, a new form of asexual reproduction in the seagrass Posidonia oceanica

Short communicationThe seagrass Posidonia oceanica is able to reproduce by asexual formation of plantlets directly in the inflorescence. Pseudoviviparous plantlets were observed during May 2004 in 26% of the inflorescences in a P. oceanica meadow off the island of Formentera (Balearic Islands, Western Mediterranean Sea). We do not know how frequently pseudovivipary occurs in Posidonia oceanica, nor the mechanisms triggering it, but this strategy can contribute significantly to short-distance dispersal and meadow maintenance, which is especially relevant in a species whose meadows can persist for thousands of years and whose colonization of new space occurs very slowly.This work was supported by the Fisheries Department (Direcció General de Pesca) of the Balearic Government.Peer reviewe

Detecting the impacts of harbour construction on a seagrass habitat and its subsequent recovery

Managing coastal development requires a set of tools to adequately detect ecosystem and water column degradation, but it also demands tools to detect any post-disturbance improvement. Structural seagrass indicators (such as shoot density or cover) are often used to detect or assess disturbances, but while they may be very sensitive to the impact itself, it is unclear if those indicators on their own can effectively reflect recovery at time scales relevant to managers. We used the construction of a harbour affecting a nearby Posidonia oceanica seagrass community to test the ability of a set of indicators (structural and others) to detect alterations and to evaluate their sensitivity to recovery of environmental quality after harbour construction was complete and the disturbance ceased. We used a Beyond Before After Control Impact (BBACI) design to evaluate effects on one impacted and three control meadows where we used structural, morphological, community and physiological indicators (26 in total) to asses disturbance impacts. Additionally, we measured some of the potential environmental factors that could be altered during and after the construction of the harbour and are critical to the survival of the seagrass meadow (light, sediment organic matter, sediment accrual). Harbour construction caused a clear increase in sediment organic matter and in sediment deposition rates, especially fine sand. Light availability was also reduced due to suspended sediments. Sediment and light conditions returned to normal levels 5 and 15 months after the construction began. As expected, seagrass structural indicators responded unequivocally to these environmental changes, with clear reductions in shoot density. Additionally, reduced light conditions quickly resulted in a decline in carbohydrate content in affected meadows. Unexpectedly, we also recorded a significant increase in metal content in plant tissues. No response was detected in the physiological indicators related to eutrophication (e.g. N and P content in tissues) and in morphological (shoot biomass) and community (epiphyte biomass) indicators. More than three years after the completion of the harbour, structural indicators did not show any sign of recovery. In contrast, physiological indicators, mainly heavy metal and carbohydrates content, were much better in detecting the improvement of the environmental conditions over the fairly short period of this study. These results indicate that while structural indicators are critical to evaluate the immediate effect of disturbances and the recovery on impacted systems, specific physiological indicators may be much better suited to determining the timing of environmental quality recovery. The design of impact and monitoring protocols in the wake of coastal developmental projects need to consider the differential effectiveness and time-response of measured indicators carefully

Evaluating potential artifacts of tethering techniques to estimate predation on sea urchins

Measuring the strength of trophic interactions in marine systems has been central to our understanding of community structuring. Sea urchin tethering has been the method of choice to evaluate rates of predation in marine benthic ecosystems. As standardly practiced, this method involves piercing the urchin test, potentially introducing significant methodological artifacts that may influence survival or detection by predators. Here we assess possible artifacts of tethering comparing invasive (pierced) and non-invasive tethering techniques using the sea urchin Paracentrotus lividus. Specifically we looked at how degree of confinement and high water temperature (first order artifacts), and predator guild and size of the prey (second order artifacts) affect the survival and/or detectability of pierced urchins. Our results show that first order artifacts only arise when pierced sea urchins are placed in sheltered bays with confined waters, especially when water temperature reaches extremely high levels. Prey detectability did not increase in pierced sea urchins for the most common predators. Also, test piercing did not alter the preferences of predators for given prey sizes. We conclude that the standard tethering technique is a robust method to test relative rates of sea urchin predation. However, local conditions could increase mortality of the tethered urchin in sheltered bays or in very high temperature regimes. Under these conditions adequate pierced controls (within predator exclusions) need to be included in assays to evaluate artifactual sources of mortality

Differences in predator composition alter the direction of structure‐mediated predation risk in macrophyte communities

Structural complexity strongly influences the outcome of predator-prey interactions in benthic marine communities affecting both prey concealment and predator hunting efficacy. How habitat structure interacts with species‐specific differences in predatory style and antipredatory strategies may therefore be critical in determining higher trophic functions. We examined the role of structural complexity in mediating predator-prey interactions across several macrophyte habitats along a gradient of structural complexity in three different bioregions: western Mediterranean Sea (WMS), eastern Indian Ocean (EIO) and northern Gulf of Mexico (NGM). Using sea urchins as model prey, we measured survival rates of small (juveniles) and medium (young adults) size classes in different habitat zones: within the macrophyte habitat, along the edge and in bare sandy spaces. At each site we also measured structural variables and predator abundance. Generalised linear models identified biomass and predatory fish abundance as the main determinants of predation intensity but the efficiency of predation was also influenced by urchin size class. Interestingly though, the direction of structure‐mediated effects on predation risk was markedly different between habitats and bioregions. In WMS and NGM, where predation by roving fish was relatively high, structure served as a critical prey refuge, particularly for juvenile urchins. In contrast, in EIO, where roving fish predation was low, predation was generally higher inside structurally complex environments where sea stars were responsible for much of the predation. Larger prey were generally less affected by predation in all habitats, probably due to the absence of large predators. Overall, our results indicate that, while the structural complexity of habitats is critical in mediating predator-prey interactions, the direction of this mediation is strongly influenced by differences in predator composition. Whether the regional pool of predators is dominated by visual roving species or chemotactic benthic predators may determine if structure dampens or enhances the influence of top-down control in marine macrophyte communities

Seguiment de la biodiversitat marina al Parc Natural de Cap de Creus i al Parc Natural del Montgrí, les Illes Medes i el Baix Ter. Informe 2014

Informe 2014. Contracte nº AG-2014-654 amb la Generalitat de Catalunya. Departament d‘Agricultura, Ramaderia, Pesca, Alimentació i Medi Natural. Servei d'Espais Naturals Protegits.Aquesta memòria presenta els resultats del grup de treball del Departament d’Ecologia de la UB respecte el seguiment de l’any 2014, tal i com consta al plec prescripcions expedient AG-2014-654, en compliment de la llei 19/1990 de 10 de desembre del Parlament de Catalunya, i amb les millores proposades a l’oferta tècnica Seguiment del medi marí als Parcs Naturals marins de Catalunya. Aquests resultats tenen l’objectiu d’avaluar les poblacions i hàbitats marins en relació tant amb les activitats humanes que hi tenen lloc com amb els factors ambientals; analitzar l’estat de les espècies i les comunitats, la seva evolució temporal i l’efecte que hi produeix la protecció, així com detectar altres situacions de risc pel patrimoni natural com podrien ser les espècies introduïdes o invasores o bé els possibles efectes del canvi climàtic