Abstract
Eutrophication causes tremendous losses to seagrass around the globe. The effects of nutrient loading vary along environmental gradients, and wave forces especially are expected to affect meadow stability, nutrient status, and responses to nutrient supply. Here, we surveyed the pristine subtropical intertidal seagrass system of Banc d’Arguin, Mauritania, to characterise Zostera noltii in terms of morphology (biomass allocation, leaf length and area, rhizome internode length), nutrient content (carbon: C, nitrogen: N, phosphorus: P, iron: Fe, aluminium: Al), and patterns in δ13C and δ15N across a wide gradient of hydrodynamic conditions. We subsequently assessed temporal variability in seagrass stability and nutrient fluxes, as well as responses to experimental fertilisation (pulses of + N, + P, + N + P) on three meadows representing different degrees of wave-force (exposed, intermediate and sheltered). The large-scale survey revealed a marked increase in N and P limitation with increasing wave energy. The overall low leaf %N (1.74 ± 0.04; mean ± se) and N:P ratio (8.67 ± 0.14) suggests that the area is N-limited. Seasonal variation in seagrass cover and biomass showed the exposed site to be the most stable and the sheltered site the least. Variation in δ15N signatures indicates seasonal shifts in N sources at the exposed site only. Fertilisation with + N and + N + P induced seagrass mortality at the exposed site, while at the sheltered site it was + P that degraded seagrass. Collectively, our results indicate that with increasing wave forces, the degree of stability of seagrass beds increases, but nutrient limitation and vulnerability to eutrophication increase as well.
Highlights
The exploitation of coastal systems has negatively affected most seagrass beds around the world (Short and Wyllie-Echeverria 1996; Orth and others 2006; Waycott and others 2009)
The objective of this study was to understand how differences in environmental context along a hydrodynamic gradient affect seagrass stoichiometry and stability, and subsequently, the sensitivity to collapse in response to nutrient pulses
There was a strong north to south gradient, ranging from 23,031 in the north to 0 J m-1 at the isolated inner intertidal flats (Figure 1B)
Summary
The exploitation of coastal systems has negatively affected most seagrass beds around the world (Short and Wyllie-Echeverria 1996; Orth and others 2006; Waycott and others 2009). Eutrophication-induced mortality is considered the most destructive factor for seagrasses (Burkholder and others 2007). Eutrophication (mainly excess nitrogen) stimulates the growth of phytoplankton, epiphytes and ephemeral macroalgae (Duarte 1995; Viaroli and others 2008), which kill seagrasses through shading and light limitation (Short and others 1995). Eutrophication generally occurs at landscape scales (Green and others 2004), sometimes leading to mass mortality of seagrasses which subsequently provokes sediment suspension (Maxwell and others 2017). The latter will further hinder seagrass recovery (Folmer and others 2012) and may trigger a regime shift (van der Heide and others 2007). Sediment dynamics may hamper longterm seagrass persistence (Suykerbuyk and others 2016)
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