Impacts of trawl fisheries on marine benthic biogeochemistry

Abstract

Fishing is widely recognised as the largest anthropogenic impact on coastal marine ecosystems. Bottom trawling is both widespread and a major impacting activity with documented effects on target species, populations of non-target species, food web dynamics and habitat features. In this thesis the effects of trawling on the benthic biogeochemistry of the central west North Sea were investigated. Two areas with similar sediment properties were identified for comparative analysis of North Sea sediments from trawled and untrawled areas. A combination of field and complimentary microcosm experiments were employed to examine the impacts of commercial trawling on the assemblage of benthic macrofauna, distributions of dissolved nutrients and chromophoric dissolved organic matter (CIDOM). The infauna in trawled areas consisted of species, the size of which were on average -36 % smaller than those from untrawled areas, whereas total mean macrofaunal abundance was -60 % greater in trawled sediments. These size and abundance differences had a profound affect on bioturbation. Natural densities of bioturbators from untrawled sediments increased benthic fluxes relative to an abiotic situation by up to 81 % NH4+i 197 % P04 3-, 96 % N02- and 33 % N03-. Fauna from trawled sediments gave flux values that lay between untrawled faunal fluxes and fluxes from controls without fauna. In addition to the long-term alteration in sediment biogeochemistry resulting from altered faunistic composition the direct impact of trawl gear produced an enhanced sediment efflux for NH4+ (475 % greater than the background flux) and N02- (26 % greater than the background flux). In contrast, P04 3- influx, observed in systems without trawl impacts changed to a net efflux following trawl disturbance (-15 %). In contrast, N03-, displayed a net efflux in the control systems, yet, a decrease of -1.0 % was apparent following heavy trawling. Altered flux rates persisted for > 48 hours. During the main fishing season nutrient profiles were perturbed down to -4 cm in sediments from trawled sites. Nutrient concentrations in the top 4 cm of trawled sediments were characterised by a relatively homogenous surface sediment layer in which concentrations were significantly lower than concentrations in untrawled sediments. This thesis has demonstrated that the impacts of commercial trawl fisheries on benthic biogeochemistry in the North Sea are of a significant magnitude and scale to affect local and potentially regional nutrient dynamics.