Abstract
Seagrass meadows form highly productive and valuable ecosystems in the marine environment. Throughout the year, seagrass meadows are exposed to abiotic and biotic variations linked to (i) seasonal fluctuations, (ii) short-term stress events such as, e.g., local nutrient enrichment, and (iii) small-scale disturbances such as, e.g., biomass removal by grazing. We hypothesized that short-term stress events and small-scale disturbances may affect seagrass chance for survival in temperate latitudes. To test this hypothesis we focused on seagrass carbon reserves in the form of starch stored seasonally in rhizomes, as these have been defined as a good indicator for winter survival. Twelve Zostera noltei meadows were monitored along a latitudinal gradient in Western Europe to firstly assess the seasonal change of their rhizomal starch content. Secondly, we tested the effects of nutrient enrichment and/or biomass removal on the corresponding starch content by using a short-term manipulative field experiment at a single latitude in the Netherlands. At the end of the growing season, we observed a weak but significant linear increase of starch content along the latitudinal gradient from south to north. This agrees with the contention that such reserves are essential for regrowth after winter, which is more severe in the north. In addition, we also observed a weak but significant positive relationship between starch content at the beginning of the growing season and past winter temperatures. This implies a lower regrowth potential after severe winters, due to diminished starch content at the beginning of the growing season. Short-term stress and disturbances may intensify these patterns, because our manipulative experiments show that when nutrient enrichment and biomass loss co-occurred at the end of the growing season, Z. noltei starch content declined. In temperate zones, the capacity of seagrasses to accumulate carbon reserves is expected to determine carbon-based regrowth after winter. Therefore, processes affecting those reserves might affect seagrass resilience. With increasing human pressure on coastal systems, short- and small-scale stress events are expected to become more frequent, threatening the resilience of seagrass ecosystems, particularly at higher latitudes, where populations tend to have an annual cycle highly dependent on their storage capacity.
Highlights
Seagrasses are flowering plants, adapted to the marine environment (Les et al, 1997), forming extensive and highly productive meadows worldwide (Short et al, 2007)
Throughout the year, temperate seagrass meadows are submitted to various abiotic and biotic variations: (i) seasonal fluctuations related to variations in light and temperature controlling their presence and seasonal growth (Dennison, 1987; Duarte, 1991; Olesen and Sand-Jensen, 1993; Ochieng et al, 2010; Marbà et al, 2012); (ii) short-term stress events such as local nutrient enrichment leading to eutrophication; and (iii) small-scale disturbances such as biomass removal by grazing, jointly affecting their resilience and survival (Burkholder et al, 2007; Macreadie et al, 2014)
Some are related to seasonal fluctuations and geographical settings; but others are rather local such as short-term nutrient enrichment or small-scale disturbances
Summary
Seagrasses are flowering plants, adapted to the marine environment (Les et al, 1997), forming extensive and highly productive meadows worldwide (Short et al, 2007). The capacity of seagrasses to respond and recover from stresses and disturbances (Charpentier et al, 2005; Godet et al, 2008) depends on their clonal growth strategy (i.e., potential rhizome elongation rate; Macreadie et al, 2014), their seasonal growth (Sordo et al, 2011; Soissons et al, 2016), their seed production (Van Tussenbroek et al, 2016), their high productivity (Plus et al, 2005; Ribaudo et al, 2016), and their ecosystem engineering capacity (McGlathery et al, 2012) Seagrasses can acclimate their morphological, physiological, and mechanical traits to local conditions (Peralta et al, 2005, 2006; Cabaço et al, 2009; de los Santos et al, 2010, 2013; La Nafie et al, 2013; Soissons et al, 2018). If changes in environmental conditions result in a decrease of seagrass resilience, their very valuable ecosystem services may decrease and disappear (Scheffer et al, 2001, 2009; Carr et al, 2012)
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