Title

The effect of natural populations of the borrowing and grazing soldier crab (Mictyris longicarpus) on sediment irrigation, benthic metabolism and nitrogen fluxes

Document Type

Article

Publication details

Webb, AP & Eyre, BD 2004, 'The effect of natural populations of the borrowing and grazing soldier crab (Mictyris longicarpus) on sediment irrigation, benthic metabolism and nitrogen fluxes', Journal of Experimental Marine Biology and Ecology, vol. 309, no. 1, pp. 1-19.

Journal of Experimental Marine Biology and Ecology journal home page available at http://www.elsevier.com/wps/find/journaldescription.cws_home/523011/description

Publisher's version of article available at http://dx.doi.org/10.1016/j.jembe.2004.05.003

Abstract

The aim of this study was to determine the effect of sediment grazing and burrowing activities of natural populations of Mictyris longicarpus on benthic metabolism, nitrogen flux and irrigation rates by comparing sediments taken from minimum disturbance exclusion cages and adjacent sediments subject to M. longicarpus activities. M. longicarpus reduced sediment surface chlorophyll a (approximately 77%), organic carbon (approximately 95%) and total nitrogen concentrations (approximately 99%) in comparison to ungrazed sediments. Consequently, they significantly reduced gross benthic O2 production (about 71%) and sediment O2 consumption (approximately 46%). Mean N2 fluxes showed net effluxes (276–430 ìmol m−2 day−1) in the presences of M. longicarpus and net uptakes (194.09–449.21 ìmol m−2 day−1) where they were excluded. The net uptake of N2 was most likely due to cyanobacteria fixing of N2, as dense microbial mats became established over the sediment surface in the absence of M. longicarpus grazing activity. Sediment irrigation/transport rates calculated from CsCl tracer dilution indicated greater irrigation rates in the exclusions (12.12–16.22 l m−2 h−1) ) compared to inhabited sediments (6.33–11.73 l m−2 h−1) and this was again was most likely due to the lack of grazing pressure which allowed large populations of small burrowing polychaetes to inhabit the organic matter rich exclusion sediments. As such, the main influence of M. longicarpus was the interception and consumption of transported organic material, benthic microalgae and other small infaunal organisms resulting in the removal of approximately 0.06 g m−2 day−1 of nitrogen and 12.12 g m−2 day−1 of organic carbon. This “cleansing” of the sediments reduced sediment metabolism and the flux of solutes across the sediment water interface and ultimately the heavy predation of M. longicarpus by transient species such as stingrays, results in a net loss of carbon and nitrogen from the system.