Exposure of coastal ecosystems to river plume spreading across a near-equatorial continental shelf

Ayi Tarya, Ton Hoitink, M. van der Vegt, Marieke van Katwijk, Bert Hoeksema, Tjeerd Bouma, Marjolijn Christianen

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

The Berau Continental Shelf (BCS) in East Kalimantan, Indonesia, harbours various tropical marine ecosystems,
including mangroves, seagrass meadows and coral reefs. These ecosystem are located partly within reach of the
Berau River plume, which may affect ecosystem health through exposure to land-derived sediments, nutrients
and pollutants carried by the plume. This study aims (1) to assess the exposure risk of the BCS coastal ecosystems
to river plume water, measured as exposure time to three different salinity levels, (2) to identify the relationships
between these salinity levels and the abundance and diversity of coral and seagrass ecosystems, and (3) to
determine a suitable indicator for the impacts of salinity on coral reef and seagrass health. We analysed hydrodynamic
models, classified salinity levels, and quantified the correlations between the salinity model parameters
and ecological metrics for the BCS systems. An Empirical Orthogonal Functions (EOF) analysis revealed
three modes of river plume dispersal patterns, which strongly reflect monsoon seasonality. The first mode,
explaining 39% of the variability, was associated with the southward movement of the plume due to northerly
winds, while the second and third modes (explaining 29% and 26% of the variability, respectively) were associated
with the northeastward migration of the plume related to southwesterly and southerly winds. Exposure
to low salinity showed higher correlations with biological indicators than mean salinity, indicating that low
salinity is a more suitable indicator for coastal ecosystem health. Significant correlations (R2) were found between
exposure time to low salinity (days with salinity values below 25 PSU) with coral cover, coral species
richness, seagrass cover, the number of seagrass species, seagrass leaf phosphorus, nitrogen, C:N ratio and iron
content. By comparing the correlation coefficients and the slopes of the regression lines, our study suggests that
coral reefs are more susceptible to low salinity levels exposure than seagrass meadows. Regarding the risk of
corals being exposed to low salinity, nearshore and northern barrier reefs were classified as “high risk”, the
middle barrier reef as “medium to high risk” and southern barrier reefs as “medium risk”. Further offshore, the
oceanic reefs were classified as “low risk”. Regarding the seagrass meadows, the nearshore region was categorized
as “high risk”, the barrier reef as “medium to low risk” and oceanic reefs as “low risk”. This study
contributes to assessing the potential impacts of salinity on the BCS ecosystems, and further provides a
knowledge base for marine conservation planning.
Original languageEnglish
Pages (from-to)1 - 15
JournalContinental Shelf Research
Volume153
DOIs
Publication statusPublished - Jan 2018

Keywords

  • river plume
  • coral reef
  • seagrass
  • hydrodynamic model
  • exposure risk

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