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Altered epiphyte community and sea urchin diet in Posidonia oceanica meadows in the vicinity of volcanic CO2 vents

Publication . Nogueira, Patrícia; Gambi, Maria Cristina; Vizzini, Salvatrice; Califano, Gianmaria; Tavares, Ana Mafalda; Santos, Rui; Martinez-Cruz, Begona

Ocean acidification (OA) predicted for 2100 is expected to shift seagrass epiphyte communities towards the dominance of more tolerant non-calcifying taxa. However, little is known about the indirect effects of such changes on food provision to key seagrass consumers. We found that epiphyte communities of the seagrass Posidonia oceanica in two naturally acidified sites (i.e. north and south sides of a volcanic CO2 vent) and in a control site away from the vent at the Ischia Island (NW Mediterranean Sea) significantly differed in composition and abundance. Such differences involved a higher abundance of non-calcareous crustose brown algae and a decline of calcifying polychaetes in both acidified sites. A lower epiphytic abundance of crustose coralline algae occurred only in the south side of the vents, thus suggesting that OA may alter epiphyte assemblages in different ways due to interaction with local factors such as differential fish herbivory or hydrodynamics. The OA effects on food items (seagrass, epiphytes, and algae) indirectly propagated into food provision to the sea urchin Paracentrotus lividus, as reflected by a reduced P. oceanica exploitation (i.e. less seagrass and calcareous epiphytes in the diet) in favour of non-calcareous green algae in both vent sites. In contrast, we detected no difference close and outside the vents neither in the composition of sea urchin diet nor in the total abundance of calcareous versus non-calcareous taxa. More research, under realistic scenarios of predicted pH reduction (i.e. <= 0.32 units of pH by 2100), is still necessary to better understand cascading effects of this altered urchin exploitation of food resources under acidified conditions on ecosystem diversity and function. (C) 2017 Elsevier Ltd. All rights reserved.

2017-06Journal article

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CO2 and nutrient-driven changes across multiple levels of organization in zostera noltii ecosystems

Publication . Martínez-Crego, Begoña; Olivé, Irene; Santos, Rui

Increasing evidence emphasizes that the effects of human impacts on ecosystems must be investigated using designs that incorporate the responses across levels of biological organization as well as the effects of multiple stressors. Here we implemented a mesocosm experiment to investigate how the individual and interactive effects of CO2 enrichment and eutrophication scale-up from changes in primary producers at the individual (biochemistry) or population level (production, reproduction, and/ or abundance) to higher levels of community (macroalgae abundance, herbivory, and global metabolism), and ecosystem organization (detritus release and carbon sink capacity). The responses of Zostera noltii seagrass meadows growing in low-and high-nutrient field conditions were compared. In both meadows, the expected CO2 benefits on Z. noltii leaf production were suppressed by epiphyte overgrowth, with no direct CO2 effect on plant biochemistry or population-level traits. Multi-level meadow response to nutrients was faster and stronger than to CO2. Nutrient enrichment promoted the nutritional quality of Z. noltii (high N, low C : N and phenolics), the growth of epiphytic pennate diatoms and purple bacteria, and shoot mortality. In the low-nutrient meadow, individual effects of CO2 and nutrients separately resulted in reduced carbon storage in the sediment, probably due to enhanced microbial degradation of more labile organic matter. These changes, however, had no effect on herbivory or on community metabolism. Interestingly, individual effects of CO2 or nutrient addition on epiphytes, shoot mortality, and carbon storage were attenuated when nutrients and CO2 acted simultaneously. This suggests CO2-induced benefits on eutrophic meadows. In the high-nutrient meadow, a striking shoot decline caused by amphipod overgrazing masked the response to CO2 and nutrient additions. Our results reveal that under future scenarios of CO2, the responses of seagrass ecosystems will be complex and context-dependent, being mediated by epiphyte overgrowth rather than by direct effects on plant biochemistry. Overall, we found that the responses of seagrass meadows to individual and interactive effects of CO2 and nutrient enrichment varied depending on interactions among species and connections between organization levels.

2014Journal article

Open Access

Effects of nutrient enrichment on seagrass population dynamics: evidence and synthesis from the biomass-density relationships

Publication . Cabaco, Susana; Apostolaki, Eugenia T.; Garcia-Marin, Patricia; Gruber, Renee; Hernandez, Ignacio; Martinez-Crego, Begona; Mascaro, Oriol; Perez, Marta; Prathep, Anchana; Robinson, Cliff; Romero, Javier; Schmidt, Allison L.; Short, Fred T.; van Tussenbroek, Brigitta I.; Santos, Rui

The available data from experimental and descriptive studies on seagrass biomass and density responses to nutrient enrichment were analysed to assess the intraspecific mechanisms operating within seagrass populations and whether biomass-density relationships can provide relevant metrics for monitoring seagrasses. The response of shoot biomass and density to nutrient enrichment was dependent on the type of study; the short-term positive response of biomass and density in experimental studies reveals context-specific nutrient limitation of seagrasses. The long-term negative response of descriptive studies probably results from ecosystem-scale events related to nutrient enrichment such as increased turbidity, algal blooms, epiphyte loads and anoxia. Most seagrass species analysed lie in the nonthinning part of the theoretical biomass-density curves. A simultaneous increase in biomass and decrease in density, evidence of self-thinning, were only observed in 4 of 28 studies. The analysis of both the static and the dynamic biomass-density relationships revealed that the slopes increase under nutrient enrichment. Surprisingly, the species-specific slopes (log B-log D) were higher than one, revealing that the B/D ratio, that is, the average shoot biomass, increases with density in all seagrass species analysed. Nutrient enrichment further enhanced this effect as biomass-density slopes increased to even higher values. The main drivers behind the increasing biomass-density slopes under nutrient enrichment were the increase in shoot biomass at densities above a species-specific threshold and/or its decrease below that threshold. Synthesis. Contrasting short- and long-term responses of both biomass and density of seagrasses to nutrient enrichment suggest that the former, positive ones result from nutrient limitation, whereas the later, negative ones are mediated by whole ecosystem responses. In general, shoot biomass of seagrasses increases with density, and nutrient enrichment enhances this effect. Experimental testing of facilitation processes related to clonal integration in seagrasses needs to be done to reveal whether they determine the low incidence of self-thinning and the intriguing biomass-density relationships of seagrass species. The increasing slopes and decreasing intercepts of the species-specific dynamic biomass-density relationships of seagrasses and the decreasing coefficients of variation of both biomass and density constitute relevant, easy-to-collect metrics that may be used in environmental monitoring.

2013-11Journal article

Open Access