Evidence for 'critical slowing down' in seagrass:a stress gradient experiment at the southern limit of its range

The theory of critical slowing down, i.e. the increasing recovery times of complex systems close to tipping points, has been proposed as an early warning signal for collapse. Empirical evidence for the reality of such warning signals is still rare in ecology. We studied this on Zostera noltii intertidal seagrass meadows at their southern range limit, the Banc d'Arguin, Mauritania. We analyse the environmental covariates of recovery rates using structural equation modelling (SEM), based on an experiment in which we assessed whether recovery after disturbances (i.e. seagrass & infauna removal) depends on stress intensity (increasing with elevation) and disturbance patch size (1 m(2) vs. 9 m(2)). The SEM analyses revealed that higher biofilm density and sediment accretion best explained seagrass recovery rates. Experimental disturbances were followed by slow rates of recovery, regrowth occurring mainly in the coolest months of the year. Macrofauna recolonisation lagged behind seagrass recovery. Overall, the recovery rate was six times slower in the high intertidal zone than in the low zone. The large disturbances in the low zone recovered faster than the small ones in the high zone. This provides empirical evidence for critical slowing down with increasing desiccation stress in an intertidal seagrass system

Sea Urchins Predation Facilitates Coral Invasion in a Marine Reserve

Macroalgae is the dominant trophic group on Mediterranean infralittoral rocky bottoms, whereas zooxanthellate corals are extremely rare. However, in recent years, the invasive coral Oculina patagonica appears to be increasing its abundance through unknown means. Here we examine the pattern of variation of this species at a marine reserve between 2002 and 2010 and contribute to the understanding of the mechanisms that allow its current increase. Because indirect interactions between species can play a relevant role in the establishment of species, a parallel assessment of the sea urchin Paracentrotus lividus, the main herbivorous invertebrate in this habitat and thus a key species, was conducted. O. patagonica has shown a 3-fold increase in abundance over the last 8 years and has become the most abundant invertebrate in the shallow waters of the marine reserve, matching some dominant erect macroalgae in abundance. High recruitment played an important role in this increasing coral abundance. The results from this study provide compelling evidence that the increase in sea urchin abundance may be one of the main drivers of the observed increase in coral abundance. Sea urchins overgraze macroalgae and create barren patches in the space-limited macroalgal community that subsequently facilitate coral recruitment. This study indicates that trophic interactions contributed to the success of an invasive coral in the Mediterranean because sea urchins grazing activity indirectly facilitated expansion of the coral. Current coral abundance at the marine reserve has ended the monopolization of algae in rocky infralittoral assemblages, an event that could greatly modify both the underwater seascape and the sources of primary production in the ecosystem

Global challenges for seagrass conservation

Seagrasses, flowering marine plants that form underwater meadows, play a significant global role in supporting food security, mitigating climate change and supporting biodiversity. Although progress is being made to conserve seagrass meadows in select areas, most meadows remain under significant pressure resulting in a decline in meadow condition and loss of function. Effective management strategies need to be implemented to reverse seagrass loss and enhance their fundamental role in coastal ocean habitats. Here we propose that seagrass meadows globally face a series of significant common challenges that must be addressed from a multifaceted and interdisciplinary perspective in order to achieve global conservation of seagrass meadows. The six main global challenges to seagrass conservation are (1) a lack of awareness of what seagrasses are and a limited societal recognition of the importance of seagrasses in coastal systems; (2) the status of many seagrass meadows are unknown, and up-to-date information on status and condition is essential; (3) understanding threatening activities at local scales is required to target management actions accordingly; (4) expanding our understanding of interactions between the socio-economic and ecological elements of seagrass systems is essential to balance the needs of people and the planet; (5) seagrass research should be expanded to generate scientific inquiries that support conservation actions; (6) increased understanding of the linkages between seagrass and climate change is required to adapt conservation accordingly. We also explicitly outline a series of proposed policy actions that will enable the scientific and conservation community to rise to these challenges. We urge the seagrass conservation community to engage stakeholders from local resource users to international policy-makers to address the challenges outlined here, in order to secure the future of the world’s seagrass ecosystems and maintain the vital services which they supply

Seasonal fish abundance and seagrass cover data, Zanzibar, Tanzania 2014-2015

<p>We conducted a field survey in four sites around Unguja Island (Zanzibar, Tanzania): two no-take MPAs (Chumbe and Mnemba Islands)  and two unprotected, fished sites (Changuu island and Mnemba reef).</p><p>The study was conducted during three seasons in 2014-2015: November-December 2014, April-May 2015 and July-August 2015. Sampling was carried out in sub-tidal seagrass beds dominated by <i>Thalassodendron ciliatum.</i></p><p>At each site, fish and seagrass sampling was conducted in 10 replicate transects (except in one of the MPAs, where only 7 transects could be sampled due to the limited size of the protected seagrass bed), which were randomly placed > 5 m apart and parallel to the shore. The surveys were carried out during high neap tide (± 3 h) through underwater visual census by snorkelling along 25×4 m belt transects. A snorkeler swam the transects twice during approx. 10 min. All fish observed within or crossing the transect were counted, identified to the lowest taxonomic level possible (usually species), and their body length was visually estimated to the nearest 5 cm. Based on the estimated sizes, all individual fish were then grouped into three age classes (juveniles, subadults and adults). The bottom per cent cover of seagrass was visually estimated within 0.25 m² quadrates placed at 5 m intervals along each transect (resulting in six subsamples per transect).</p

Seasonality vs. Protection. Seasonality effects on seagrass fish within and outside MPAs

<p>Seasonality effects on seagrass fish within and outside MPAs</p

Fishing disrupts the herbivory–plant growth link in seagrasses

Seasonality effects on herbivory-plant growth link in protected and unprotected tropical seagrass bed