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The lack of top‐down control on Tripneustes gratilla, a sea urchin commonly known to graze on seagrass, and the bottom‐up control of its feeding preference, led to the overgrazing of seagrass meadows of the species Thalassodendron ciliatum in Changuu Island (Zanzibar Archipelago). The impact of overgrazing on seagrasses was assessed by mapping the presence of grazed versus non‐grazed seagrass patches in the study site, while the top‐down control on T. gratilla was assessed by measuring the abundance of its fish predators. The feeding preference and distribution of T. gratilla were characterized by calculating the electivity indexes for each seagrass species and measuring sea urchin density, respectively. Approximately half of the patches of T. ciliatum were overgrazed, while predatory fishes of T. gratilla were absent from the site. The Vanderploeg and Scavia's Relativized Electivity Index indicated that T. gratilla had a feeding preference for T. ciliatum, which was also supported by higher urchin densities within T. ciliatum dominated patches. Bottom‐up control of grazing activity was observed by quantifying and analyzing morphological, nutritional, and the chemical defense traits of the seagrass in relation to feeding preference and urchin density. Feeding was positively correlated to the seagrass tissue C:P ratio (ρ = 0.9), whereas urchin density showed no correlations. The bottom‐up control of the feeding preference and agglomeration of T. gratilla in T. ciliatum meadows, together with the lack of evidence of substantial top‐down control and the long recovery time of T. ciliatum led to the overgrazing of this species at this site. Overgrazing, therefore, was shown to be the result of multiple factors ranging from the traits of the seagrass and feeding preference of T. gratilla, to the abundance of predators in this area. [ABSTRACT FROM AUTHOR]

Eutrophication is one of the main threats to seagrass meadows, but there is limited knowledge on the interactive effects of nutrients under a changing climate, particularly for tropical seagrass species. This study aimed to detect the onset of stress in the tropical seagrass, Halophila stipulacea , by investigating the effect of in situ nutrient addition during an unusually warm summer over a 6-month period. We measured a suite of different morphological and biochemical community metrics and individual plant traits from two different sites with contrasting levels of eutrophication history before and after in situ fertilization in the Gulf of Aqaba. Nutrient stress combined with summer temperatures that surpassed the threshold for optimal growth negatively affected seagrass plants from South Beach (SB), an oligotrophic marine protected area, while H. stipulacea populations from North Beach (NB), a eutrophic and anthropogenically impacted area, benefited from the additional nutrient input. Lower aboveground (AG) and belowground (BG) biomass, reduced Leaf Area Index (LAI), smaller internodal distances, high sexual reproductive effort and the increasing occurrence of apical shoots in seagrasses from SB sites indicated that the plants were under stress and not growing under optimal conditions. Moreover, AG and BG biomass and internodal distances decreased further with the addition of fertilizer in SB sites. Results presented here highlight the fact that H. stipulacea is one of the most tolerant and plastic seagrass species. Our study further demonstrates that the effects of fertilization differ significantly between meadows that are growing exposed to different levels of anthropogenic pressures. Thus, the meadow's "history" affects it resilience and response to further stress. Our results suggest that monitoring efforts on H. stipulacea populations in its native range should focus especially on carbohydrate reserves in leaves and rhizomes, LAI, internodal length and percentage of apical shoots as suitable warning indicators for nutrient stress in this seagrass species to minimize future impacts on these valuable ecosystems. [ABSTRACT FROM AUTHOR]

Posidonia oceanica is a key foundation species in the Mediterranean providing valuable ecosystem services. However, this species is particularly vulnerable towards high coastal nutrient inputs and the rising frequency of intense summer heat waves, but their combined effect in situ has received little attention so far. Here, we investigated the effects of in situ nutrient addition during an unusually warm summer over a 4-month period, comparing different morphological, physiological and biochemical population metrics of seagrass meadows growing in protected areas (Ischia) with meadows already exposed to significant anthropogenic pressure (Baia – Gulf of Pozzuoli). Our study highlights that the effects of warmer than usual summer temperatures on the population level of seagrass meadows can be exacerbated if the plants are already exposed to higher anthropogenic pressures. Morphological and population level indicators mainly changed over time, possibly impacted by season and the warmer temperatures, and displayed more pronounced reductions in seagrasses from impacted sites. The additional nutrient supply had even more deleterious effects, as shown by a decrease in approximately 67% in cover in fertilized plots at high impacted sites and 33% at low impacted sites. Moreover, while rhizome starch concentration showed a seasonal increase in plants from low impacted sites it displayed a trend of a 27% decrease in fertilized plots of the high impacted sites. Epiphyte biomass was approximately four-fold higher on leaves of plants growing in impacted sites and even doubled with the additional nutrient input. Predicting and anticipating stress in P. oceanica is of crucial importance for conservation and management efforts, given the limited colonizing and reproductive ability and extremely slow growth of this ecosystem engineer. Our results suggest that monitoring efforts should focus especially on leaf area index (LAI), carbohydrate concentrations in the rhizomes, and epiphyte cover on leaves as indicators of the onset of stress in Posidonia oceanica , which can be used by decision makers to take appropriate measures before damage to the ecosystem becomes irreversible, minimize future human interference and strengthen the resilience of these important ecosystems. [ABSTRACT FROM AUTHOR]

"Nutrient dynamics in seagrass ecosystems", in Seagrasses: Biology, Ecology and Conservation. Climate change, eutrophication, responses of seagrasses to single and combined stressors, seagrasses, spatial-temporal modeling Keywords: seagrasses; climate change; eutrophication; responses of seagrasses to single and combined stressors; spatial-temporal modeling EN seagrasses climate change eutrophication responses of seagrasses to single and combined stressors spatial-temporal modeling 1 3 3 05/02/22 20220428 NES 220428 Awareness of the ecological importance of seagrasses is growing due to recent attention to their role in carbon sequestration as a potential blue carbon sink (Fourqurean et al., [2]; Bedulli et al.), as well as their role in nutrient cycling (Romero et al., [9]), sediment stabilization (James et al., [4]), pathogen filtration (Lamb et al., [6]), and the formation of essential habitats for economically important marine species (Jackson et al., [3]; Jones et al.). [Extracted from the article]

During the last 150 years, the tropical seagrass species Halophila stipulacea has established itself in the southern and eastern parts of the Mediterranean Sea. More recently (2018), Halophila decipiens was observed for the first time in the eastern Mediterranean, and was described as the second non-native seagrass species in the Mediterranean Sea. We implemented a species distribution model (SDM) approach to (1) hindcast the habitat suitability of H. stipulacea over the last 100 years in the Mediterranean basin, and (2) to model the increase in the potential habitat suitability of H. stipulacea and H. decipiens during the current century under two very different climate scenarios, RCP 2.6 (lowest carbon emission scenario) and RCP 8.5 (highest carbon emission scenario). In addition, a principal component analysis (PCA) and k -means cluster based on temperature and salinity drivers were applied to visualize the distance and relatedness between the native and invasive H. stipulacea and H. decipiens populations. Results from this PCA suggest that the H. stipulacea populations of the Mediterranean and Red Sea are likely to be similar. In contrast, H. decipiens from the Mediterranean is more related to the Atlantic populations rather than to the Red Sea populations. The hindcast model suggests that the expansion of H. stipulacea was related to the increases in seawater temperatures in the Mediterranean over the last 100 years. The SDMs predict that more suitable habitat will become available for both tropical species during this century. The habitat suitability for H. stipulacea will keep expanding westward and northward as the Mediterranean continues to become saltier and warmer. In comparison, the SDMs built for H. decipiens forecast a restricted habitat suitability in the south-eastern Mediterranean Sea at the present environmental conditions and predicts a progressive expansion with a potential increase in habitat suitability along 85% of the Mediterranean coastline. The predicted rapid expansion of non-native seagrass species could alter the Mediterranean's seagrass community and may entail massive impacts on associated ecosystem functions and services, impacts that have severe socio-economic consequences. [ABSTRACT FROM AUTHOR]

The international demand for macroalgae products has increased driving the expansion of seaweed farming to many coastal rural areas worldwide. Yet it is not fully understood what the impacts of this practice are on local habitats, including seagrass meadows, one of the most productive coastal ecosystems. Additionally, high nutrient inputs to coastal systems via direct freshwater run-off or groundwater can cause algal blooms with negative consequences to seagrasses and seaweed production. This study aims to detect the short term ecological impacts of seaweed farming on seagrass meadows, as well as the influence of nutrient inputs via groundwater on seaweed productivity in the near-shore aquatic system of Rote Island, Indonesia. Seaweed farming plots were set up over seagrass meadows in two locations on the island with assumed higher and lower anthropogenic pressure in Nemberala village and Tunggaoen beach, respectively. Seaweed production rates, seagrass indices, and environmental indicators were measured within the seaweed farm and in plots directly outside of the farm over two seaweed growth cycles. Seaweed production was positively affected by environmental changes during the rainy season in the region, including lower temperatures, higher water motion and higher nutrient concentrations. On the other hand, results show 15% reduction in seagrass shoot density, as well as reduced leaf length, plant biomass, and percentage cover under the seaweed farms. Nuisance algae were found attached or entangled in the seaweed fronds, especially in Nemberala, causing further light reduction under the farms. The impacts on seagrasses also led to diminution of C org (22-23%) and nitrogen (22-29%) in surface sediment in the plots compared to unfarmed areas. δ15N of the suspended particulate matter of sampled groundwater from the beach seepages in both sites were high, 13.4‰ in Nemberala and 17.8‰ in Tunggaoen, indicating heavier sources of N either from sewage contamination or isotopic discrimination due to microbial processing of groundwater. During the rainy season, the N- and P-enriched water entering the system led to an increase in nutrient concentrations in Nemberala bay seawater, causing eutrophication. [Display omitted] • Seaweed farming negatively impacted seagrass density, percent cover and biomass. • Carbon and nitrogen sequestration capacity of seagrass habitats was reduced under seaweed farming. • Eutrophication from land-borne nutrients enhanced seaweed farming impacts on meadows. • Sustainable seaweed practices are recommended to better protect seagrass habitats. [ABSTRACT FROM AUTHOR]