Bouma, T.J.

Seagrass beds are highly productive coastal ecosystems providing a large array of ecosystem services including !sheries and carbon sequestration. As seagrasses are known to be highly sensitive to anthropogenic forcing, we evaluated the use of trace metal concentrations in seagrasses as bioindicators for trace metal pollution of coastal regions at both global and local scale. We carried out a meta-analysis based on literature data to provide a global benchmark list for trace metal accumulation in seagrasses, which was lacking in literature. We subsequently carried out a case study at the Caribbean islands of Curaçao and Bonaire to test for local-scale differences in trace metal concentrations in seagrasses, and
internal metal allocation. The benchmark and local study show that trace metal concentrations in seagrass leaves, regardless of the species, can vary over a 100e1000-fold range, and are related to the level of anthropogenic pressure, making seagrasses highly valuable indicators.

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Seagrass meadows provide a spectrum of anthropogenically important ecological functions. However, these ecosystems are globally declining under human influence. Submerged communities inhabiting different water motion regimes are experiencing different stressors and are likely to respond differently to climate change. This study couples a comparison of conditions between water motion regimes with indications of susceptibility to stochastic disturbances and investigates both their influences on seagrass meadow community structure. Community surveys were conducted in a semi-structured manner at three different wave-exposed sites on Saint Martin, French West Indies. Measurements of mass transfer and other abiotic features were taken alongside. Nutrients, pH, salinity, light, temperature and DIC could not explain dissimilarities in macrophyte communities between sites. Multivariate ordination methods in which the bareness of the seafloor was used as a gradient, gave indications of setbacks to earlier successional stages in seagrass meadow communities. Indirect assessments of grazing pressure yielded sea urchins as potential herbivores in studied meadows, having a possibly profound effect on altering species composition, but probably having no effect on sea floor cover. The wave-exposed community showed more bareness in between macrophytes. Exposed sites, which are prone to experience higher rates of stochastic disturbances, probably need calcifying algae to stabilize sediment for recolonization after meadow losses. Penicillus capitatus and Halimeda spp., both rhizophytic calcifying algae, were found to be the most probable colonizers of bare seafloor. The role of these species as supposed colonizer, and therefore establisher or facilitator of seagrass meadows should be experimentally investigated, as these algae are thought to suffer competitive disadvantages under future ocean acidification scenarios. 

Seagrasses comprise 78 species and are rarely invasive. But the seagrass Halophila stipulacea, firstly recorded in the Caribbean in the year 2002, has spread quickly throughout the region. Previous works have described this species as invasive in the Caribbean, forming dense mats that exclude native seagrass species. During a reconnaissance field survey of Caribbean seagrass meadows at the islands of Bonaire and Sint Maarten in 2013, we observed that this species was only extremely dense at 5 out of 10 studied meadows. Compared to areas with sparse growth of H. stipulacea, these dense meadows showed consistently higher nutrient concentrations, as indicated by higher leaf tissue N contents of the seagrass Thalassia testudinum (dense when C:N < 22.5) and sediments (dense when %N > 11.3). Thus, the potential invasiveness of this non-native seagrass most likely depends on the environmental conditions, especially the nutrient concentrations.

Seagrass beds are globally declining due to human activities in coastal areas. We here aimed to identify threats from eutrophication to the valuable seagrass beds of Curaçao and Bonaire in the Caribbean, which function as nursery habitats for commercial fish species. We documented surface- and porewater nutrient concentrations, and seagrass nutrient concentrations in 6 bays varying in nutrient loads. Water measurements only provided a momentary snapshot, due to timing, tidal stage, etc., but Thalassia testudinum nutrient concentrations indicated long-term nutrient loads. Nutrient levels in most bays did not raise any concern, but high leaf % P values of Thalassia in Piscadera Bay (0.31%) and Spanish Water Bay (0.21%) showed that seagrasses may be threatened by eutrophication, due to emergency overflow of waste water and coastal housing. We thus showed that seagrasses may be threatened and measures should be taken to prevent loss of these important nursery areas due to eutrophication.