J.M. Gee

Effects of organic enrichment on meiofaunal abundance and community structure in sublittoral soft sediments

In experimental mesocosms established at Solbergstrand, Oslofjord, Norway, organic enrichment was effected by the addition of powdered Ascophyllum nodosum (L.) Le Jol., in quantities equivalent to 50gC·m−2 and 200gC·m−2, to boxes of sublittoral soft sediment. After 56 days, the structure of the meiofaunal communities in these treatments was compared with that of the control boxes. At this time the meiofaunal communites at each level of organic enrichment were markedly different from each other and from that in the control sediment. The responses of the two major components of the meiofauna, however, were different. Although the abundance of nematodes was slightly reduced in the high dose treatment this was not accompanied by detectable changes in community structure. Harpacticoid copepods, on the other hand, increased significantly in abundance in the treatment boxes and showed a general trend towards increased dominance and decreased diversity with increasing levels of organic enrichment, although in the low dose treatment there was also an increase in the number of species present. It is also shown that the nematode/copepod ratio is unreliable as a biomonitoring tool and it is suggested that the differential responses in community structure between the nematode and copepod components of the meiofauna might be a better indication of stress at the community level.

Determination of fine-scale vertical distribution of microbes and meiofauna in an intertidal sediment

A simple sampling device is described which produces thin (1 mm) sections of sediment cores. The sampler has been tested on fine sand of an intertidal sandflat and used to study the vertical distribution, over part of a tidal cycle in August, 1981, of migrating algae in the surface 20 mm of sand. Two species of Diplonies and one of Navicula showed marked changes in vertical distribution as the sandflat was flooded, but the distribution of bacteria in the sime samples did not show any change with tidal state. Spatial separation of different species of harpacticoid oppepods within the surface 20 mm of sand has also been demonstrated using this sampler, and the results suggest that different species may occupy particular fine-scale spatial niches within the sand column. The depth separation of nematode species was less well defined, except for two species with apparently the same feeding mode which were isolated from one another vertically.

Field experiments on the role of epibenthic predators in determining prey densities in an estuarine mudflat

A series of caging experiments was performed on an estuarine mudflat at three seasons of the year, in which Carcinus maenas L. and Pomatoschistus microps (Kroyer) were either excluded from, or allowed to prey upon, the benthos in order to determine to what extent infaunal abundance and mortality was a result of predation by epibenthic predators. The difficulties of conducting and interpreting the results of such experiments are recognized. The benthic macrofauna of this mudflat is dominated numerically by small annelids and there is evidence that adult C. maenas can cause significant increases in the oligochaete component of this assemblage, probably as a result of disturbance caused by its burrowing activity. Juvenile C. maenas on the other hand significantly reduced the abundance of small annelids, particularly the dominant polychaete Manayunkia aestuarina (Bourne) and could be responsible for year-to-year variations in abundance of this species. The role of fish predators (in this case P. microps) is more problematical but it is suggested that in the densities at which they occur naturally on the mudflat they have little direct effect on the abundance of prey species. There is no evidence that seasonal mortality of small annelids is reduced in the absence of predation and this is taken to indicate that not all mortality is due to epibenthic predation. Certain changes in relative abundance of the component species of the harpacticoid copepod community were discerned but it is suggested that the plasticity of their reproductive potential is such that the effect of predation on the group as a whole is usually masked.

Faunistic control of Enteromorpha blooms: A field experiment

Gobies, Pomatoschistus microps (Kroeyer), were introduced to a series of cages on an estuarine mud-flat during the period in which the cover of Enteromorpha sp. was declining. They fed selectively on the grazers which either ingested Enteromorpha directly or browsed on micro-organisms attached to it and, therefore, significantly affected the breakdown rate of the alga. It is suggested that the balance between grazers and their predators may be at least partially responsible for the irregular development of Enteromorpha on estuarine mud-flats from year to year.

Effects of the feeding activity of the polychaete Streblosoma bairdi (Malmgren) on meiofaunal abundance and community structure

Streblosoma bairdi forms prominent faecal mounds in sublittoral mud mesocosms established at Solbergstrand, Oslofjorden, Norway. The abundance of meiofauna (nematodes and copepods) is highest in the mounds and very reduced in the surrounding feeding area, with intermediate densities in unaffected areas. Species diversity is highest in the feeding area, with a more equitable distribution of species, while the mounds and unaffected areas have higher dominance and lower diversity. It is suggested that these differences arise partly because of varying levels of predation, disturbance, and sediment modification resulting from the feeding activity of the polychaete. However, differences in the trophic structure of the nematode component of the meiofauna between the three areas suggest that diversity profiles are also modified indirectly by the influence of the polychaete on the nature of the primary food resources available to the meiofauna. The importance of macrofauna/ meiofauna interactions in controlling meiofaunal community structure is emphasized.

Preliminary observations on the metabolic and reproductive strategies of harpacticoid copepods from an intertidal sandflat

The metabolic responses to temperature changes and the reproductive strategies of four species of harpacticoid copepod are examined in relation to their depth distribution, susceptibility to predation and assumptions about food supply. Two species, one of which is heavily preyed upon, are confined to the surface layers and two species occur deeper in the sediment through most of the year. These latter species appear to have adopted different metabolic strategies in response to lowered food supply but both have a lower Q10 than surface dwelling species. In addition the surface species have the ability to regulate their respiratory demands at times of high, short-term, temperature fluctuations. Preliminary data suggests that generally, the surface species have shorter development times than those dwelling deeper in the sediment but until more information is available on the energetic costs of reproduction it cannot be shown that deeper dwelling species are more conservative in their reproductive strategies.

Population dynamics of Mytilicola intestinalis in Mytilus edulis in south west England

The population dynamics of Mytilicola intestinalis Steuer in mussels (Mytilus edulis L.) from the River Lynher, Cornwall, England, have been studied over 3 years. By transplanting uninfested mussels from the River Erme, South Devon, into the Lynher mussel bed, the study was extended to the growth and development of new infestations under natural conditions. Female Mytilicola intestinalis are shown to breed twice, and two generations of parasites coexist for most of the year, with recruitment taking place in summer and autumn. One generation contributes its first brood to the autumn recruits before overwintering and contributing its second brood to the following summers recruits. The other generation overwinters as juvenile and immature stages to contribute its two broods successively to the summer and autumn recruits. Environmental temperatures are believed to control the rates of development at all stages rather than acting as triggers in the onset or cessation of breeding at specific times. There is no evidence to support the contention that heavily infested mussels are killed, and parasite mortality is shown to be density-independent.