The seasonal growth pattern of the seagrass Posidonia oceanica (L.) Delile was examined in 5 meadows in NE Spain to assess the relative importance of large-scale versus local factors in controbng the seasonal patterns observed. Large-scale seasonal forcing, resulting from changes in light and temperature associated with the solar cycle, was assessed from the coherence of seasonal growth palterns among the meadows and accounted for 46 and 43% of variability in shoot size and growth, respectively. The local component of seasonality, which results from local variation in environmental variables (e.g. nutrients, dissolved inorganic carbon, redox potential) was assessed as the differences in the annual time course of shoot size and growth among the meadows, and accounted for 9 and 1 2 % of the variability, respectively. These results support the contention that seagrass seasonality is primarily controlled by the solar cycle, and secondarily by seasonal changes in the environment which are at leas1 in part caused by the temporal variability of seagrass growth. This indirect link between light and temperature and local conditions needs to be taken into account to interpret correlations between such variables and seagrass growth.

The rates of seagrass defoliation exerted by the herbivorous fish Sarpa salpa and by the sea urchin Paracentrotus lividus were evaluated through both direct (tethering experiment) and indirect (bite marks) methods. Sampling was conducted once per season in 10 shallow meadows of Posidonia oceanica (L.) Delile from the continental NW Mediterranean coast covering a spatial scale of > 300 km. Results indicated that a large proportion (ca. 57%) of the annual leaf production is lost to herbivory, yet with considerable spatial variation. Patterns of seagrass defoliation showed high temporal variability, with a peak in summer with values that exceeded about 2.5 times those of leaf production and a minimum during the winter period. On average, defoliation exerted by S. salpa accounted for 40% of leaf production (ca. 70% of total annual losses to herbivory), while P. lividus was also responsible for a substantial 17% removal of leaf production. High discrepancies encountered when comparing direct and indirect measurements suggest that the latter are inappropriate to achieve accurate estimates of herbivory pressure. This study evidences that P. oceanica leaf losses to herbivores are not marginal, but a widespread process that occurs at much higher rates than previously estimated through indirect methods (ca. 2%), resetting the paradigm of the negligible importance of herbivory in temperate systems.

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We present the seasonal carbon (C) balance of the Mediterranean seagrass Posidonia oceanica (L.) Delile calculated from seasonal rates of C gain (photosynthesis), C loss (respiration) and growth. We compare our balance with the evolution of seasonal C reserves in order to determine the parameters (shoot:root biomass, reserve allocation, photosynthetic parameters, etc.) that influence the seasonal cycle of the plant. Additionally, we examine whether the annual C balance can be used as a valid tool for testing the vulnerability of seagrasses to light reduction. The seasonal whole-plant C balance showed alternate negative (from September to June) and positive (July and August) values. This trend was the result of the interplay among several seasonal factors such as irradiance, water turbidity, photosynthetic parameters, respiratory rates, shoot growth, within-shoot age distribution, and principally, the low photosynthetic:non-photosynthetic biomass ratio. The lack of significant correlation between seasonal growth and metabolic balance (C gain -C demand) did not permit the prediction of plant growth. Conversely, the seasonal pattern of carbon storage was consistent with the periods of positive and negative C balance. Consequently, reserve mobilization allows overwintering and re-growth under conditions of negative C balance. Using different calculations the annual C balance was found to be negative during 1993; this is in accordance with the carbohydrate interannual depletion and the shoot density decline. Since Posidonia oceanica is regressing in the Mediterranean, our carbon budget may notably contribute to future carbon models that can be essential tools for defining the minimum light requirements for survival. More insight into the functioning of some of the parameters that definitively influence this carbon budget (e.g.: the rhizome/root oxygen consumption and the O 2 to C conversion) is needed to fully understand the vulnerability of seagrasses to light reduction.

Seagrass meadows, key ecosystems supporting fisheries, carbon sequestration and coastal protection, are globally threatened. In Europe, loss and recovery of seagrasses are reported, but the changes in extent and density at the continental scale remain unclear. Here we collate assessments of changes from 1869 to 2016 and show that 1/3 of European seagrass area was lost due to disease, deteriorated water quality, and coastal development, with losses peaking in the 1970s and 1980s. Since then, loss rates slowed down for most of the species and fast-growing species recovered in some locations, making the net rate of change in seagrass area experience a reversal in the 2000s, while density metrics improved or remained stable in most sites. Our results demonstrate that decline is not the generalised state among seagrasses nowadays in Europe, in contrast with global assessments, and that deceleration and reversal of declining trends is possible, expectingly bringing back the services they provide.

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Efforts to conserve globally declining herbivorous green sea turtles have resulted in promising growth of some populations. These trends could significantly impact critical ecosystem services provided by seagrass meadows on which turtles feed. Expanding turtle populations could improve seagrass ecosystem health by removing seagrass biomass and preventing of the formation of sediment anoxia. However, overfishing of large sharks, the primary green turtle predators, could facilitate turtle populations growing beyond historical sizes and trigger detrimental ecosystem impacts mirroring those on land when top predators were extirpated. Experimental data from multiple ocean basins suggest that increasing turtle populations can negatively impact seagrasses, including triggering virtual ecosystem collapse. Impacts of large turtle populations on seagrasses are reduced in the presence of intact shark populations. Healthy populations of sharks and turtles, therefore, are likely vital to restoring or maintaining seagrass ecosystem structure, function, and their value in supporting fisheries and as a carbon sink.

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