Tropical seagrass beds are productive coastal ecosystems that are important blue carbon sinks and crucial habitats and feeding grounds for consumers at high trophic levels. To understand how energy sustains the ecosystem from seagrass production, we constructed an Ecopath trophic model to reveal the possible pathways of energy flow in the tropical seagrass beds around Dongsha Island, South China Sea. The model indicates that Dongsha seagrass beds were developing but well-structured ecosystems. The productive seagrasses were rarely directly consumed by herbivores and, ultimately, flowed into detrital pools. Detritus was the main food source used to support diverse consumers in the food web. Nevertheless, the low cycling rate (2.74%) suggests that most detritus was not reused or exported and was stored as a large organic carbon pool in the sediments. The detritus-feeding invertebrates are keystone groups in the Dongsha seagrass beds, as they recycle energy from detritus and transfer to top predators such as sharks. The predation of top predators affected the biomasses of other compartments, leading to strong top-down control via a trophic cascade effect.

Seagrass beds serve as important carbon sinks, and it is thought that increasing the quantity and quality of such sinks could help to slow the rate of global climate change. Therefore, it will be important to (1) gain a better understanding of seagrass bed metabolism and (2) document how these high-productivity ecosystems are impacted by climate change-associated factors, such as ocean acidification (OA) and ocean warming (OW). A mesocosm-based approach was taken herein in which a tropical, Western Pacific seagrass species Thalassia hemprichii was cultured under either control or OA-simulating conditions; the temperature was gradually increased from 25 to 31 °C for both CO₂ enrichment treatments, and it was hypothesized that this species would respond positively to OA and elevated temperature. After 12 weeks of exposure, OA (~1200 ppm) led to (1) increases in underground biomass and root C:N ratios and (2) decreases in root nitrogen content. Rising temperatures (25 to 31 °C) increased the maximum quantum yield of photosystem II (Fv:Fm), productivity, leaf growth rate, decomposition rate, and carbon sequestration, but decreased the rate of shoot density increase and the carbon content of the leaves; this indicates that warming alone does not increase the short-term carbon sink capacity of this seagrass species. Under high CO₂ and the highest temperature employed (31 °C), this seagrass demonstrated its highest productivity, Fv:Fm, leaf growth rate, and carbon sequestration. Collectively, then, it appears that high CO₂ levels offset the negative effects of high temperature on this seagrass species. Whether this pattern is maintained at temperatures that actually induce marked seagrass stress (likely beginning at 33–34 °C in Southern Taiwan) should be the focus of future research.

The effects of seagrass on microalgal assemblages under experimentally elevated temperatures (28°C) and CO₂ partial pressures (pCO₂; 800 μatm) were examined using coral reef mesocosms. Concentrations of nitrate, ammonium, and benthic microalgal chlorophyll a (chl-a) were significantly higher in seagrass mesocosms, whereas phytoplankton chl-a concentrations were similar between seagrass and seagrass-free control mesocosms. In the seagrass group, fewer parasitic dinoflagellate OTUs (e.g., Syndiniales) were found in the benthic microalgal community though more symbiotic dinoflagellates (e.g., Cladocopium spp.) were quantified in the phytoplankton community. Our results suggest that, under ocean acidification conditions, the presence of seagrass nearby coral reefs may (1) enhance benthic primary productivity, (2) decrease parasitic dinoflagellate abundance, and (3) possibly increase the presence of symbiotic dinoflagellates.

Biogeographic patterns for marine fishes and scleractinian stony corals are well-established, but it is still unclear whether these patterns apply to the understudied marine invertebrates. Here we investigate the distribution of zoantharians (=zoanthids), an order of cnidarians that are distributed globally from tropical coral reefs to deep temperate oceans. We examined the available literature, along with new records and morphological and DNA sequence data, to establish their first global biogeographical assessment. Two striking results emerged that contrast with previously known marine biogeographical patterns. The first was that several zoantharian genera are cosmopolitan, inhabiting most biogeographic provinces within and between ocean basins. Although the highest diversity was observed in the Indo-Pacific Ocean (IPO), concordant with biogeographic assays of reef fishes and stony corals, the distribution of genera was not significantly different between the Atlantic (ATO) and the IPO. Secondly, there were multiple sibling species complexes between the ATO and the IPO. At species and population scales, a long pelagic larval duration, asexual reproduction modes, and rafting abilities, may at least partly explain the low levels of genetic differences between ocean basins. As some zoantharian species can play a significant role in phase shifts, filling knowledge gaps on species distributions is essential to support monitoring of reef ecosystems. Our study revealed that several Zoantharia genera, and most families, are globally distributed. This is highly distinct from the pattern previously observed for common reef animals, such as stony corals and fishes, where the Indo-Pacific Ocean has several times the number of familes/genera compared to the Atlantic Ocean. Some zoantharian species, especially in the genera Palythoa and Zoanthus , have dispersal strategies that allow for broad distributions within ocean basins, while the vicariant events of the rise of the Isthmus of Panama and the appearance of the Benguela upwelling likely prompted the speciation of sibling lineages between the oceans.