Spatial and temporal variation in the elemental and stable isotopic content of the seagrasses Posidonia oceanica and Cymodocea nodosa from the Illes Balears, Spain

Morphology, elemental content and isotopic composition of leaves of the seagrasses Posidonia oceanica and Cymodocea nodosa were highly variable across the Illes Balears, a Spanish archipelago in the western Mediterranean, and varied seasonally at one site in the study area. The data presented in this paper generally expand the reported ranges of nitrogen, phosphorus, iron and arsenic content and δ13C and δ15N for these species. Nitrogen and phosphorus content of P. oceanica leaves also showed significant seasonal variability; on an annual basis, P. oceanica leaves averaged 1.55% N and 0.14% P at this monitoring site. Both N and P were more concentrated in the leaves in winter than in summer, with winter maxima of 1.76% N and 0.17% P and summer minima of 1.34% N and 0.11% P. There was no significant annual pattern observed in the δ13C of P. oceanica leaves, but there was a repeated 0.6‰ seasonal fluctuation in δ15N. Mean annual δ15N was 4.0‰; δ15N was lowest in May and it increased through the summer and autumn to a maximum in November. Over the geographic range of our study area, there were interspecific differences in the carbon, nitrogen and phosphorus content of the two species. Posidonia oceanica N:P ratios were distributed around the critical value of 30:1 while the ratios for C. nodosa were lower than this value, suggesting P. oceanica we collected was not consistently limited by N or P while C. nodosa tended toward nitrogen limitation. Nutrient content was significantly correlated to morphological indicators of plant vigor. Fe content of P. oceanica leaves varied by a factor of 5×, with a minimum of 31.1 μg g−1 and a maximum of 167.7 μg g−1. Arsenic was present in much lower tissue concentrations than Fe, but the As concentrations were more variable; the maximum concentration of 1.60 μg g−1 was eight times as high as the minimum of 0.20 μg g−1. There were interspecific differences in δ13C of the two species; C. nodosa was consistently more enriched (δ13C = −7.8 ± 1.7‰) than P. oceanica (−13.2 ± 1.2‰). The δ13C of both species decreased significantly with increasing water depth. Depth related and regional variability in the δ13C and δ15N of both species were marked, suggesting that caution needs to be exercised when applying stable isotopes in food web analyses

The roles of seagrasses in structuring associated fish assemblages and fisheries

Seagrasses are known to provide important habitats for a diversity of fish and fisheries species. Continued research has allowed us to re-evaluate the generalisations, and identify the gaps in our knowledge regarding these habitats, particularly in an Australian context. Seagrasses generally form part of a mosaic with other habitats within a seascape that contributes to its overall biodiversity of fish. Patterns of abundance and diversity of fish between seagrass and other habitats, such as unvegetated flats and reef habitats, is inconsistent and depends on the region, fish and seagrass species, and sampling method. Edge effects, adjacent habitats, and fragmentation can strongly influence fish assemblages. Seagrass structural complexity can enhance survival and growth of juvenile fishes, but recent studies show that survival rates of individual prey do not vary greatly across seagrass densities when densities of both prey and predators increase with seagrass density. The concept of the nursery habitat has been built on data from studies in estuaries or highly seasonal seagrass habitats, whereas recent studies in marine systems or cool temperate seagrass meadows suggest that this role does not always hold. Direct grazing on seagrasses by fishes occurs mainly in tropical regions, although there is a paucity of data on this process along with several other processes, from tropical Australia. Grazing on seagrasses by fishes appears to be limited in temperate regions, with consumption of seagrass restricted mainly to omnivorous species. However, tropicalisation, that is, the immigration of tropical grazers to higher latitudes due to global ocean warming, is predicted to increase grazing rates on temperate seagrasses. Reductions in seagrass biomass caused by increased grazing will disrupt connectivity processes between seagrass meadows and surrounding habitats, and are likely to have significant ramifications for the biodiversity and ecosystem services those other coastal habitats provide. Although other habitats rely on inputs of seagrass detritus, and the immigration of fish and fisheries species from their juvenile seagrass habitats, quantitative data on this link are limited. Evidence that fisheries declines, either directly or indirectly, have resulted from seagrass loss is equivocal to date, and therefore, the quantification of this role is still needed. Managing seagrass for fisheries is complex, and many fisheries agencies embrace ecosystem-based management, but do not have direct responsibility for seagrass habitat. Significant progress has been made in our knowledge of fish and fisheries in seagrasses, but our review highlights significant knowledge gaps where further research is recommended