Colin Little

Subaerial exposure and changes in the stability of intertidal estuarine sediments

Abstract The surface cohesive strength of intertidal sediments at six sites on two estuaries throughout the period of a single diurnal subaerial exposure was investigated. Stations at different tidal heights were examined at each site. Hourly measurements were made using a cohesive strength meter which provides an ordinal measure of sediment stability. The sediment was most easily eroded immediately after the emersion of the mudflat and there was little difference between the initial stability of the mud at different sites or stations. As the subaerial exposure continued the sediment of mid-shore and high-shore stations began to increase in stability. No significant change was found at low-shore stations before the return of the tide. The surface density of diatom populations was also intermittently measured at each site. The greatest increase in sediment stability occurred at high-shore stations and this was most extreme where there were also dense populations of epipelic diatoms present at the surface of the sediment. This supports laboratory evidence that the extracellular products of diatom populations of sufficient density increase the stability of intertidal sediments.

Faunal distributions in soft sediments of the Severn Estuary

Abstract The pattern of the infauna on participate shores of the southern coast of the Severn Estuary has been examined. Although mud predominates as the major beach substratum within the Estuary, sand beaches occur in both the lower and upper reaches. Liquid mud is found at l.w. in the middle reaches. Suitability of substratum is shown to be the most important factor influencing animal distribution. The number of infaunal species found in this survey did not increase markedly towards the mouth, in contrast to the pattern displayed by rocky shore animals. Several species were rare or absent on mud beaches close to the entrance of the River Avon and the possibility that sewage contamination is responsible for this is discussed.

Foraging behaviour of Patella vulgata L. in an Irish sea-lough

Abstract The foraging movements of limpets Patella vulgata L. were studied on a vertical rock face at Lough Hyne, County Cork, Ireland. Rhythms on two adjacent grids were similar, but showed some differences in timing. Distance moved and speed of movement were related to limpet size. Feeding excursions were more regular and prolonged on spring tides than on neaps. Limpets high on the shore were active ony at night, at low water, when they moved vertically up the rock face. They fed in habitats unlike those surrounding their homes, mainly on rock that was either bare, or covered by lichens. Limpets near mid-tide level did not move preferentially in any one direction, and they appeared to show no choice of feeding area. Limpets low on the shore were active near the times of low water both at night and during the day. They tended to move horizontally and, at least on some occasions, they chose areas of low barnacle density in which to feed. The feeding patterns seen at different tidal heights are discussed in relation to growth rates.

Pogonophora: Uptake of Dissolved Nutrients

THE mechanism by which the marine tube-dwelling worms of the phylum Pogonophora obtain their food has remained obscure since Ivanov1 established that they have no conventional gut. Ivanov concluded that they are filter feeders, utilizing external digestion, while Carlisle2 has suggested that particles are actively picked up by the tentacles of the unitentaculate forms. Jägersten3 first put forward the idea that they absorb organic substances directly from solution, and this report provides evidence in support of his hypothesis.

The structure of benthic Diatom assemblages: A preliminary account of the use and evaluation of low-temperature scanning electron microscopy

Abstract Diatom assemblages on the surface of fine estuarine sediments have, for the first time, been examined undisturbed by a novel application of low-temperature SEM. The technique allows the visualization of surface diatoms and determination of the composition and orientation of the exposed assemblages at any one time. In a laboratory experiment, the total number of diatoms counted was similar to that obtained using the standard lens tissue technique. Although identification was more accurate using the light microscope, many different species could be readily distinguished with low-temperature SEM. An index of similarity analysis suggested a difference in the composition of the assemblages measured by the two techniques. This difference was considered to be due to the selectivity of the lens tissue technique. Further avenues of research in this subject using low-temperature SEM are briefly outlined.

The burrowing and floating behaviour of the gastropod Hydrobia ulvae

Abstract The behaviour of Hydrobia ulvae has been investigated at several localities on the British coast. No evidence of rhythmic behaviour patterns, previously postulated from laboratory studies, has been found. Floating appears to be an accidental phenomenon, brought about by a variety of mechanisms. It may aid species dispersal.

Patterns of foraging activity in the limpet Patellavulgata L. — A preliminary study

Abstract Activity of limpets was studied at Lough Ine, Co. Cork, Ireland. The small tidal range allowed long periods of continuous observation over consecutive tidal cycles. All activity was associated with the time of low water, but showed varying relations to it depending on vertical height on the shore and on the time in the spring/neap cycle. Low-water limpets were predominantly active in the day, but also showed a minor activity peak on the low water at night. Limpets higher on the shore were active only at night. It is possible that these differences reflect different behaviour in relation to the effects of desiccation and predation at different levels on the shore. The activity periods were shown to be exclusively concerned with feeding: radula rasping was almost continuous while the limpets were away from the home scar. Limpets at different levels had, however, different behaviour patterns in terms of the areas that they used for grazing.

Vertical zonation on rocky shores in the Severn estuary

Zonation of algae and invertebrates was examined on rocky shores in the Severn estuary from 1975 to 1978. These shores were characterised by an absence of macro-algae from MLWS and below. Some littoral algae showed reductions in abundance during the study. In particular, Ascophyllum nodosum and Fucus serratus almost disappeared from the mid shore at one site after 1976, and no significant recolonization had occurred by 1979. Animal species in general showed predictable and constant zonation patterns, but Littorina rudis, Patella vulgata and Elminius modestus all exhibited decreased upper vertical limits in the upper reaches of the estuary. Three approaches were taken to interpret the zonation patterns and their changes. (i) Exposure to wave action was estimated. Decreasing exposure levels may account for the decline in upper vertical limits of species in the upper estuary. (ii) A survey of silt, salinity and temperature was carried out over complete tidal cycles. High silt concentrations may partially explain the absence of macro-algae below MLWS. (iii) Preliminary experiments compared the effects of grazers in the estuary with those in fully marine conditions. These experiments point to some of the factors which are responsible for the lack of algal recolonization on the shore from which the dominant fucoids disappeared after 1976, but the situation is complex and involves algal reproductive biology as well as grazer activity.

Foraging activity of limpets in normal and abnormal tidal regimes

Activity of Patella vulgata was monitored in the asymmetrical tidal regime of an Irish sealough, Lough Hyne, and also outside the lough in a normal tidal regime. An automated method was used, allowing continuous records to be made over two weeks. Most activity occurred at night while the limpets were emersed. Rainfall depressed activity. There was little activity during daytime emersion, contrary to the results of previous studies in which low-shore limpets foraged diurnally as well as nocturnally. Timing of activity in relation to tidal coverage was similar inside the lough and outside. Activity of high-shore limpets within the lough was greater at spring tides than at neaps, but that of low-shore limpets was greater at neaps. Outside the lough, both high-shore and low-shore limpets showed greater activity at spring tides. Reasons for the differences are discussed. Using Maximum Entropy Spectral Analysis, three periodicities underlying limpet activity were indicated. These were at h, 124 h and 8–2 h. When limpet activity was simulated by adding three sine waves of appropriate periodicity, rhythms very similar to those recorded from the shore were produced.

Decadal-scale changes in the shallow subtidal benthos of an Irish marine reserve.

Abstract Long-term annual monitoring of rocky intertidal and shallow subtidal shores of an Irish sea lough (1990–2010) documented major regime shifts in the past decade. When population densities of the purple urchin Paracentrotus lividus plummeted in Lough Hyne Marine Reserve in southwestern Ireland, the warm-water fucalean alga Cystoseira foeniculacea and ephemeral algae proliferated shortly after. We discuss the possible influences of release from herbivory and climate change on this algal proliferation, which blanketed the benthos. Smothering of the benthos led to high levels of shallow subtidal anoxia. Furthermore, the invasive fucalean alga Sargassum muticum has made repeated incursions into the reserve. Although being reduced by persistent eradication efforts (2003–2011), Sargassum is spreading within the lough. Limited seawater flushing and propagule dispersal within the lough and eutrophication in coastal waters may have contributed to community-level changes. Whether the regime change is cyclical (contingent on re-establishment of urchins within the lough and continued eradication of S. muticum) or irreversible (the result of critical transitions) remains unclear.