Seagrass Sensitivity to Collapse Along a Hydrodynamic Gradient: Evidence from a Pristine Subtropical Intertidal Ecosystem

El Hacen M. El-Hacen*, Tjeerd J. Bouma, Laura L. Govers, Theunis Piersma, Han Olff

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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.

Original languageEnglish
Pages (from-to)1007-1023
Number of pages17
JournalEcosystems
Volume22
Issue number5
DOIs
Publication statusPublished - 15 Aug 2019

Keywords

  • Banc d’Arguin-Mauritania
  • environmental gradient
  • eutrophication
  • fertilisation
  • hydrodynamic
  • nutrient limitation
  • pristine environment
  • Zostera noltii

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