Document of bibliographic reference 291118

BibliographicReference record

Type

Bibliographic resource

Type of document

Book/Monograph

Type of document

Dissertation

BibLvlCode

M

Title

Sedimentary response to sea cucumber (Holothuria scabra) removal: Insights from experimental manipulations on a Fijian reef

Abstract

Bioturbation through reef ecosystem engineers such as sea cucumbers can potentially enhance recycling of organic matter (OM) in reef sands. Benthic-pelagic coupling demonstrates that changes to the water column influence sediment properties as well as that sedimentary processes can affect the quality of overlying water. Coastal ecosystems are increasingly stressed by OM enrichment from human activities such as farming and sewage discharge. Thus the ability of sediment to degrade OM efficiently is vital to the buffering capacity of coastal ecosystems. However, growing demand from Asian markets is driving the overexploitation of sea cucumbers. Consequently, there is a need to understand the impact of their removal on reef ecosystems. This thesis aims to develop an understanding of how the removal of sea cucumbers affects sedimentary function in a biocatalytical recycling system. Holothuria scabra was historically found in high densities and exhibit a natural burrowing cycle thus playing a key role in bioturbation. Densities of H. scabra were manipulated in enclosures in situ on a reef flat at Natuvu Village on Vanua Levu, Fiji to simulate different fishing intensities. Two treatments (n=4 treatment-1) were used; high density (350 g m-2) and exclusion (0 g m-2), and two controls accounted for cage effects; cage control (no cage walls) and natural (60 g m-2). Sedimentary oxygen consumption (SOC), grain size distribution, porosity, and oxygen (O2) penetration depth were recorded. Sediment reworked through ingestion by H. scabra was quantified over the course of 2 weeks. After incorporating spatial surveys of H. scabra, our calculations show that the current population on Natuvu’s reef flat has the potential to rework ca. 7850 kg sediment dry wt year-1 1000 m-2. SOC rates were consistently higher in exclusion than high-density enclosures. O2 penetration depth decreased significantly when the stressors of both sea cucumber removal and elevated sea temperatures (likely caused by the 2016 El Niño) were combined. Thus removal of sea cucumbers may reduce the capacity of reef sediment to buffer negative effects of OM pulses, impeding the function and productivity of reef ecosystems. Consequently Pacific island communities are left more vulnerable as the ecosystem services that they rely heavily on for their livelihood may be compromised as a result of the extensive overfishing of sea cucumbers.

Bibliographic citation

Lee, S.D.A. (2016). Sedimentary response to sea cucumber (Holothuria scabra) removal: Insights from experimental manipulations on a Fijian reef. MSc Thesis. University of Bremen, Faculty for Biology and Chemistry: Bremen. 71 pp.