Deposit-Feeding Sea Cucumbers Enhance Mineralization and Nutrient Cycling in Organically-Enriched Coastal Sediments

Thomas Mactavish,Candida Savage,Kay Vopel,Jeanie Stenton-Dozey,Martin Solan

doi:10.1371/journal.pone.0050031

Abstract

BackgroundBioturbators affect multiple biogeochemical interactions and have been suggested as suitable candidates to mitigate organic matter loading in marine sediments. However, predicting the effects of bioturbators at an ecosystem level can be difficult due to their complex positive and negative interactions with the microbial community.Methodology/Principal FindingsWe quantified the effects of deposit-feeding sea cucumbers on benthic algal biomass (microphytobenthos, MPB), bacterial abundance, and the sediment–seawater exchange of dissolved oxygen and nutrients. The sea cucumbers increased the efflux of inorganic nitrogen (ammonium, NH4 +) from organically enriched sediments, which stimulated algal productivity. Grazing by the sea cucumbers on MPB (evidenced by pheopigments), however, caused a net negative effect on primary producer biomass and total oxygen production. Further, there was an increased abundance of bacteria in sediment with sea cucumbers, suggesting facilitation. The sea cucumbers increased the ratio of oxygen consumption to production in surface sediment by shifting the microbial balance from producers to decomposers. This shift explains the increased efflux of inorganic nitrogen and concordant reduction in organic matter content in sediment with bioturbators.Conclusions/SignificanceOur study demonstrates the functional role and potential of sea cucumbers to ameliorate some of the adverse effects of organic matter enrichment in coastal ecosystems.

Highlights

Bioturbators enhance ecosystem functioning in marine sediments by modifying geochemical gradients, redistributing food resources [1], and altering nutrient fluxes
Porosity of the top 0.5 cm of sediment was significantly higher in cores with added mussel biodeposits (SM-cores) and biodeposits plus sea cucumbers (SMSC-cores) compared to initial (I-cores) and control cores (S-cores) (Table 1)
Cores), with 10.460.7% recorded in the top 0.5 cm after 14 days (Figure 1). This was significantly greater than organic matter (OM) content in the cores with mussel feces and sea cucumbers (SMSC-cores) for the same period (7.960.7%) (Table 1)

Summary

Introduction

Bioturbators enhance ecosystem functioning in marine sediments by modifying geochemical gradients, redistributing food resources [1], and altering nutrient fluxes. Global invertebrate catches have increased six-fold since 1950 [5], with overfishing causing population declines in 81% of sea cucumber fisheries [5], a ubiquitous and functionally important group of bioturbators [1]. Given their important role as bioturbators, it has been suggested that the presence of deposit-feeding sea cucumbers can reduce the accumulation of excess organic matter in coastal sediments such as under aquaculture farms [7,8]. Predicting the effects of bioturbators at an ecosystem level can be difficult due to their complex positive and negative interactions with the microbial community

Methods

Results

Discussion

Conclusion