Patterns of change in coral reef communities of a remote Maldivian atoll revisited after eleven years.

Coral reefs are exposed worldwide to several global and local human pressures including climate change and coastal development. Assessing the effects of such pressures on coral reef communities and the changes they undergo over time is mandatory to understand their possible future trends. Nonetheless, some coral reefs receive no or little scientific attention, as in the case of Huvadhoo Atoll that is an under-studied region in the southernmost area of the Maldives (Indian Ocean). This study analyzes the changes occurring over time in eight coral reefs (four inner reefs within the atoll lagoon and four outer reefs on the ocean side) at Huvadhoo Atoll, firstly surveyed in 2009 and revisited in 2020 using the same field methods. The cover of 23 morphological benthic descriptors (including different growth forms of Acropora) was taken into account and then grouped into three categories (i.e., hard coral, other benthic taxa and abiotic descriptors) to analyze the change in the composition of the coral reef community. Significant changes (e.g., increase in hard coral cover and decrease in abiotic descriptors) were observed in the inner reefs as compared to the outer reefs, which showed less variability. A significant decrease in tabular Acropora cover was observed in both inner and outer reefs, with possible negative effects on reef complexity and functioning. By comparing two time periods and two reef types, this study provides novel information on the change over time in the community composition of Maldivian coral reefs.

Unexpected slow recovery of seagrass leaf epiphytes after the impact of a summer heat wave and concomitant mucilage bloom.

The epiphytes of seagrass leaves constitute a peculiar community, comprised of a number of species specialized for this living substrate. Several studies report on the response of epiphytes to different pressures but no information exists about the effects of summer heatwaves, which have become frequent events in the last decades. This paper represents the first attempt to investigate the change in the leaf epiphyte community of the Mediterranean seagrass Posidonia oceanica due to the heatwave occurred in summer 2003. Thanks to a series of data collected seasonally between 2002 and 2006, and punctual data in the summers of 2014 and 2019, we assessed the change over time in the leaf epiphyte community. Temperature data trends were analysed through linear regression, while multivariate analyses (i.e., nMDS and SIMPER) were applied to cover data in order to assess changes over time in the epiphyte community. As a whole, the two most abundant taxa were the crustose coralline alga Hydrolithon and the encrusting bryozoan Electra posidoniae, which displayed the highest average cover values in summer (around 19%) and spring (around 9%), respectively. Epiphytes proved to be sensitive to temperature highs, displaying different effects on cover, biomass, diversity and community composition. Cover and biomass exhibited a dramatic reduction (more than 60%) after the disturbance. In particular, Hydrolithon more than halved, while E. posidoniae dropped sevenfold during summer 2003. While the former recovered comparatively quickly, the latter, as well as the whole community composition, apparently required 16 years to return to a condition similar to that of 2002.

Serpulid reefs and their role in aquatic ecosystems: A global review.

The Serpulidae are a large family of sedentary polychaetes, characterized by a calcareous habitation tube, which they cannot leave. The calcium carbonate tube is in the form of both aragonite and calcite, in fairly constant ratio for each taxon. Tubes are cemented firmly to any hard substrate (in only few species tubes are free). Although in the majority of the species the tubes encrust the substrate for all their length, the distal part may eventually detach and grow erectly. Certain species in dense populations build tubes vertical to the substrate in clumps and cement the tubes to each other. This gives serpulids the capability of forming reef-life structures when densely settling. Despite the relative smallness of the individual tubes (rarely longer than 15cm and wider than 1cm), such reef-like structures may cover tens of m2, with a layer more than 1m thick. Serpulid reefs can be divided roughly into seven groups, according to the building modality and the type of habitat they occupy: (i) pseudocolonies; (ii) littoral belts; (iii) subtidal to deep-water reefs; (iv) reefs in coastal lakes and harbours; (v) brackish water reefs; (vi) tapestries in freshwater caves; (vii) biostalactites inside marine caves. The role of serpulid reefs in the ecosystems they inhabit is multifarious and may be distinguished in functions (biomass and production, benthic pelagic coupling, resistance and resilience, reproductive and survivorship strategies, trophodynamics, bioconstruction, living space and refuge, nursery, sediment formation and retention, food for other species, carbonate deposition and storage) and services (water clearance, reef associated fishery, cultural benefits). On the other hand, many serpulids are important constituents of biological fouling, and their calcareous masses damage submerged artefacts, causing huge economic costs. Positive and negative roles of serpulid reefs need to be compared with common metrics; the overall balance, however, is still to be assessed.

Variability between observers does not hamper detecting change over time in a temperate reef.

Marine ecosystems are subject to global and local impacts, both contributing to dramatic changes in coastal communities. Assessing such changes requires time series or the revisitation of sites first surveyed in the past. In both cases, data are not necessarily collected by the same observers, which could lead to a bias in the results. In the Marine Protected Area (MPA) of Capo Carbonara (Sardinia, Italy), established in 1998, rocky reef communities were first assessed in 2000 by two diving scientists. Twenty years later, the same rocky reefs were resurveyed using the same method by two other diving scientists. In both surveys, semi-quantitative data on conspicuous species were collected at five sites in four depth zones, providing the possibility of assessing change over time. To explore the influence of climate and local pressures, existing data on sea surface temperature, resident population, tourism and diving activities were analysed. The reef communities of the Capo Carbonara MPA have distinctly changed over time, mostly under the effect of seawater warming, as highlighted by the occurrence of thermophilic species and by other climate-related indicators. On the other side, species vulnerable to local human pressures have increased over time, demonstrating the effectiveness of the protection measures undertaken by the MPA. Comparing data collected by four different observers in the two periods demonstrated that change over time was significantly greater than variability between the observers.