Effects of megafaunal/macrofaunal burrowing interactions on benthic community structure

Smith, Christopher John (1988) Effects of megafaunal/macrofaunal burrowing interactions on benthic community structure. PhD thesis, University of Glasgow.

Abstract

Megafaunal burrowing organisms are, in comparison to the macrofauna, mostly sparsely distributed, inhabiting deep sedimentary levels below regular sampling depth. They have been little studied until recent advances in sampling methodology. As large scale bioturbators, they have a high potential impact on the sedimentary environment and its inhabitants. This thesis assesses both the role of megafaunal burrowers in controlling macrofaunal community structure, and some of the mechanisms involved. Megafaunal burrowers are also subject to controlling forces which may directly or indirectly affect the macrofaunal community. Two of these factors, organic enrichment and fishing pressure were investigated. Four "typical" west coast of Scotland sub-littoral inshore soft sedimentary sites were surveyed: Loch Creran, under the influence of an organic effluent from an alginate factory; Loch Riddon, under the influence of general eutrophication; Camas Nathais, under fishing pressure for the megafaunal burrower Nephrops norveqicus, and Creag Isle, taken as a relatively undisturbed "control" site. Megafaunal, macrofaunal and sedimentary characteristics were analysed. Organic enrichment was responsible for high numbers of relatively immobile surface feeding macrofaunal species at the two former sites. The control site had the highest megafaunal abundance with an increase in the sediment/water interface of 27%, from the internal surface area of large burrows. The predominant nonstochastic factors that seem to determine megafaunal community structure are a complex interaction of sedimentary characteristics, organic carbon content and, to a lesser extent, depth. These factors, especially organic carbon content, were also important in regulating macrofaunal community structure. Megafaunal burrowers were further responsible for localized structuring. The localized effect of burrower/burrow presence was investigated on a fine scale in the field by diver coring around the burrow of Nephrops norvegicus and in an adjacent unburrowed area. The burrowed area had a more homogeneous sedimentary fabric with sedimentary depth, indicative of vertical mixing. Redox potentials and organic carbon content showed greater variability with sedimentary depth. Macrofaunal abundance and biomass were lower in this area and this was reflected by the low numbers of surface feeding, discretely motile and tubiculous polychaetes. In the experimental aquaria, mesocosm tanks were used to substantiate the field observations and to further investigate the mechanisms of megafaunal effects. Different burrowers were found to cause both detrimental and enhancing effects which were dependent on their activity and life mode. Nephrops norveqicus, a large surface-active burrower, had a more inhibitive effect. It excluded surface feeding species by increased turbidity and burial, and generally inhibited macrofaunal activity. However, lack of competition and continual perturbation allowed colonization around the burrowed area by a small opportunist population of nematodes and oligochaetes. Calocaris macandreae, a less active burrower that remains subsurface, had a less marked detrimental effect on macrofaunal community structure. It is suggested that this burrower attracted a number of species by providing organic rich excavated sediment (for example, Corbula qibba, Turritella communis, Maqelona filiformis and Paraonis gracilis. Megafaunal abundance was investigated by remote video along an organic gradient at the Garroch Head sludge dumping ground in the Clyde. Abundance decreased with increasing organic content of the sediment in towards the centre of the ground. It is suggested that the major causes were due to sedimentary modification from increased fine material, effects on juvenile forms and general anaerobic sedimentary conditions. The response of different species varied and was related to their tolerance to the above- mentioned factors. The implication of megafaunal absence is discussed in relation to macrofaunal community structure and it is suggested that macrofaunal changes along an organic enriched gradient may be more abrupt than expected at the point where megafaunal species are excluded. The role of biological interactions in determining macrofaunal community structure is reviewed. Megafaunal interactions, which include aspects of competition, predation, disturbance and enhancement, are discussed with respect to the established theories. It is suggested that megafaunal burrowing organisms have an important role in increasing spatio-temporal heterogeneity in temperate marine soft sedimentary communities.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Biological oceanography, Ecology
• Date of Award: 1988
• Depositing User: Enlighten Team
• Unique ID: glathesis:1988-76704
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Dec 2019 09:15
• Last Modified: 19 Dec 2019 09:15
• URI: https://theses.gla.ac.uk/id/eprint/76704