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Recent declines in coastal water quality in SW Ireland have led to changes in the abundance of algal-associated bryozoans and a hydroid. These common North Atlantic species offer considerable potential as indicator species for reduced oxygen conditions due to habitat degradation. Annual monitoring for more than two decades (1994–2016) at 10 rocky shore sites within Lough Hyne Marine Reserve, SW Ireland revealed a significant reduction in abundance of the ctenostome bryozoan, Flustrellidra hispida, through time, coincident with the development of daily, extreme, shallow water oxygen fluctuations (hyperoxia and hypoxia) due to eutrophication. In contrast, the ctenostome, Alcyonidium species-complex, increased significantly during the two-decade period. The thecate hydroid, Dynamena pumila, did not show any significant decadal-scale pattern but temporal variability in abundance increased since 2010. Abundances of F. hispida and D. pumila were positively correlated with estimates of water flow. F. hispida inhabited a variety of algal hosts but appeared most sensitive (of the taxa surveyed) to variation in habitat quality, especially experimentally reduced levels of dissolved oxygen ( −1 ). Such sensitivity indicates F. hispida could be useful as an indicator of environmental change, whereas D. pumila was more robust in response to reduced oxygen with 38.6% of the hydroid zooids being able to survive under hypoxic conditions as compared to only 0.2% of the bryozoan zooids. Given the widespread European distribution of these species, they offer the opportunity to act as important bioindicators of water quality and hence environmental degradation in north temperate intertidal systems.

Long-term cyclical changes in rocky shore community structure were documented over two decades at sheltered shores in Lough Hyne Marine Reserve, County Cork, Ireland. Three peaks of abundance were recorded for the limpet, Patella vulgata (1990-5, 2002-6 and 2010-14) with oscillations that varied in amplitude and frequency among sites. The cover of the fucoid Fucus vesiculosus varied inversely with limpet abundance and showed the strongest negative correlation with a lag time of 0–2 years. The species complex Fucus spiralis/guiryi showed a weaker correlation with a lag time of 1–2 years. Two other fucoid species showed no such negative correlations despite their close proximity to limpets within the lough's compressed tidal range. There was no relationship between overall barnacle cover (dominated by Austrominius modestus ) and the limpet-fucoid cycles, suggesting that the shelter provided by A. modestus for algae to escape from limpet grazing pressure may not be necessary for these cycles to occur on wave-sheltered shores.

The benthos of Lough Hyne (Europe’s first marine reserve) in SW Ireland has changed dramatically in the past decade with declining invertebrate communities and proliferating ephemeral macroalgae. In summer 2011 and 2012, we measured dissolved oxygen (DO) levels on shallow subtidal rocky shores of the lough and recorded both benthic hypoxia (

A twenty-five year (1994–2018) survey at 10 rocky sites within Lough Hyne Marine Reserve (SW Ireland) showed that, despite considerable spatial and temporal patchiness, three barnacle species (Chthamalus stellatus, Semibalanus balanoides, and Austrominius modestus) declined in overall abundance in the early 2000s, particularly at three sites. There was no clear response of A. modestus or C. stellatus to the cold winters of 2010 and 2011. In 2004, however, S. balanoides declined suddenly, but showed a slight recovery after the cold winter of 2010 to maintain sparse populations in the lough, but remained abundant outside the lough. A fourth species, Chthamalus montagui, showed a different pattern, with wide fluctuations in abundance but no overall decline. Changes in the Atlantic Multidecadal Oscillation may have influenced the overall composition of barnacle populations. Rising maximum air temperatures, resulting in high body temperatures recorded for A. modestus even during mild days, are suggested to have impacted the barnacle populations as a result of the prolonged aerial exposure times caused by the lough's asymmetric tidal pattern.

Two-decade-long monitoring studies at Europe's first statutory marine reserve—Lough Hyne in SW Ireland—indicate that benthic communities are rapidly changing. Populations of the ecologically important purple urchin ( Paracentrotus lividus ) have fluctuated widely, most recently with a population boom in the late 1990s, followed by a mass mortality that persists to the present day. Eight general hypotheses have been proposed to account for the urchin decline including cold temperature limiting reproduction, ephemeral algal exudates disrupting urchin fertilization, low larval availability (due to over-harvesting and/or episodic recruitment), high mortality of settlers and juveniles due to hypoxia, hyperoxia or predation (a trophic cascade hypothesis), and increased mortality due to pathogens (stress hypothesis). The cold-temperature and the trophic cascade hypotheses appear unlikely. The remaining hypotheses, however, all seem to play a role, to some degree, in driving the urchin decline. Ulvoid exudates, for example, significantly reduced urchin fertilization and few larvae were found in plankton tows (2012–2015), indicating low larval availability in summer. Whilst settling urchins regularly recruited under shallow-subtidal rocks until 2011, no settlers were found in these habitats from 2011 to 2014 or in field experiments (2012–2018) using various settlement substrata. Seawater quality was poor in shallow areas of the lough with extreme oxygen fluctuations (diel-cycling hypoxia), and 1-day experimental exposures to DO values −1 were lethal to most juvenile urchins. Multiple increases of the predatory spiny starfish ( Marthasterias glacialis ) population in recent decades may also have contributed to the demise of the coexisting juvenile urchins. Finally, urchins of all sizes were seen suffering from dropped spines, tissue necrosis, or white-coloured infection, suggestive of stress-related pathogen mortality. There was a paucity of broken tests, indicating limited predation by large crustaceans; the large number of adult urchins ‘missing’ and few P. lividus tests on the north shore points to possible urchin removal by poachers and/or starfish predation. While these ecological, environmental, and anthropogenic processes occur on many open coast rocky shores, many are exacerbated by the semi-enclosed nature of this fully marine sea lough due to its limited flushing. Multiple factors, including low larval availability and rapidly expanding starfish populations, coupled with degraded habitat quality (ephemeral algal mats and extreme oxygen fluctuations), indicate that the purple urchin populations will not recover without an improvement in the water quality of Lough Hyne Marine Reserve, the restocking of urchins and protection from poaching.

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.

Both Hydrobia ulvae (Pennant) and Potamopyrgus jenkinsi Smith climb vertically in the laboratory but stop or crawl horizontally if presented with a sharp salinity drop. 50% of Hydrobia ulvae respond to a difference of 18–26% from the original solution. 50% of Potamopyrgus jenkinsi respond to a difference of 53–55%. In both species, the response is impaired at high temperatures, and in Hydrobia ulvae it is also impaired at low temperatures. From 9–22 °C no temperature influence is evident. From acclimation experiments it is suggested that the snails respond not to the gradient across the interface, but to the gradient between body fluids and the upper solution presented. Snails respond to non-ionic gradients when these are presented either as density gradients or as solutions separated by a semi-permeable membrane, and it is argued that they detect osmotic pressure directly. The sense organs are probably located in the tentacles.

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 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.

Growth and survival of veligers of Amphibola crenata (Gmelin) were followed in a range of salinities from 4 to 35%. In salinities of 18%. and below, growth was slowed and a large proportion of veligers died. In 4 to 12%., no larvae survived to metamorphosis, but these low salinities were tolerated for short periods. Newly post-metamorphic snails and juveniles collected from the field survived well even at 4%., showing that greater tolerance of low external salinities developed at metamorphosis. Examination of the physiology of adult snails suggested that this may be due to the development of osmoregulating mechanisms, because the adults maintain the blood hyperosmotic to the medium in 4 and 9%. The kidney, however, does not seem to be involved in this regulation because urine is isosmotic with the blood. An extra-renal mechanism of salt uptake is, therefore, postulated.

Population densities of Nais elinguis Muller were determined in Swanpool, a brackish-water lagoon at Falmouth, Cornwall, U.K., over a four-year period. High densities were found only from January to May, usually with a peak in March. Significant negative correlations were shown between population density and both salinity and temperature. In laboratory tests, feeding rates remained unchanged from freshwater to 20‰ salinity (S), but declined above this salinity. Nais elinguis was shown to be a good osmoregulator, remaining hyperosmotic below 7‰ S, and hypo-osmotic above this. Feeding rate showed a Q10 of approximately 2 from 1 to 25°C, but above this the rate declined. Feeding rate was unaffected between pH 6 and 11. Increased salinity to (10‰ S) did not influence the effect of temperature on feeding rate. This high salinity did reduce feeding rate at a pH of 10 and above. It is concluded that the physical and chemical variables considered are unlikely to be direct causal factors limiting populations of N. elinguis in Swanpool. The influence of food supply, competition, predation and changes in reproductive mode are discussed as possible controlling factors. It is shown that the population decline of N. elinguis in early summer usually coincides with the rise of populations of chironomid larvae.

Nutrient levels, plant pigments, oxygen balance and phytoplankton populations have been studied over a period of 2 years in a brackish-water lagoon. In general the lagoon was found to be supplied with abundant nutrients for plant growth by its freshwater inflow stream. Being shallow, it was therefore frequently eutrophic in summer. The number of phytoplankton species found in the pool was low, but several species showed marked continuity over the study period. Phytoplankton diversity was low, as often found in eutrophic lakes, and on occasions extremely low owing to the dominance of Gyrodinium resplendens. This species formed dense blooms which showed uneven horizontal and vertical distributions. In early summer, G. resplendens accumulated in strongly stratified layers of water about 1·5 m below the surface and caused maxima of oxygen and oxygen production at this level. Over the period of study some marked changes of species composition occurred, but these did not conform to any clear seasonal pattern such as that typical of many temperate freshwater lakes.

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.

The organization of the Aufwuchs community in a brackish-water lagoon (Swanpool, Falmouth, U.K.) is described. Changes in the population densities of encrusting bryozoans and mobile meiofauna are described for a period of 3 years. Most meiofaunal species reached peak densities in the spring (January–March). These included tardigrades ( Macrobiotus sp.), oligochaetes ( Nais elinguis, Chaetogaster diaphanus ), the harpacticoid copepod Schizopera clandestina , ostracods, the nematodes Dichromadora geophila and Theristus spp., and possibly the nematodes Chromadorina germanica and Atrochromadora microlaima . Other meiofaunal populations peaked in summer (July–September), and these included the chironomid Chironomus salinarius , the harpacticoid copepod Nitocra spinipes and the nematode Adoncholaimus thalassophygas . Two further species, the mite Halacarus balticus and the nematode Aphelencoides sp., showed irregular bursts in numbers. It is concluded that the spring-peaking species increased in numbers dependent upon the growth of the Aufwuchs, and particularly of the surface film of diatoms, while the summer-peaking species may have been controlled more by limiting values of salinity and temperature. These conclusions are contrasted with the general view of salinity as the over-riding factor in brackish-water ecosystems.

Variations in invertebrate densities within the Severn Estuary have been examined on four shores over a period of two years. Variability due to seasonal changes was combined with a patchy distribution of some species, while others showed consistent densities at the same tidal level and at different sampling times. Some species showed more consistent densities in winter than in summer. Biological interactions, varying periods of spawning and recruitment and beach instability are thought to be three of the major factors influencing distribution. No marked community changes such as seen in some sublittoral areas have been found.

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.

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.

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.

The fine structure of the integument in several species of Pogonophora has been examined by electron microscopy. The cuticle over the main body is composed of several layers of orthogonally arranged fibres embedded in an amorphous matrix. It is regularly traversed by microvilli from underlying epidermal cells. Toothed bristles of the annuli and setae of the anchor are composed of closely packed fibrous cylinders wrapped in a cortical material. In fine structure the cuticle, setae, toothed bristles (or setae) and setal sacs forming the setae closely resemble the corresponding structures in annelids. The cuticle is maximally thick over the forepart (protosome + mesosome) ; it is very thin and non-fibrous over the surface of the metameric papillae and over extensive areas of post-metameric trunk. The possibilities of a collagenous nature of the cuticle fibres and their mode of secretion by the epidermal cells are discussed. The organization of various cell-types forming the epidermis over the entire animal is examined. Possible functions of these cell-types are discussed. Notable amongst these are ‘possible zymogen cells’ and some absorptive cells. The intriguing question of nutrition in these gut-less tubiculous animals is re-examined in the light of present observations.

Swanpool at Falmouth, Cornwall has a mean depth of 1.6 m and contains about 80 000 m3 of brackish water. It is estimated that in one year 20 to 30 times this volume of fresh water and about 2 pool-volumes of sea water enter the pool. The fresh-water flow is continuous, while the sea water enters (through the culvert by which brackish water normally flows out of the pool) only near high water of spring tides. The pool is about 5.05 m above chart datum as compared with the mean high water neap level of 4.2 m and the mean high water spring level of 5.3 m. Consequently the smaller spring tides in midsummer, and even some in midwinter, contribute little or no sea water to the pool, which is liable to become unusually dilute at these times. A well-defined halocline normally persists throughout the year; only once in the five years of our observations has the composition of the water been uniform and at that time the chlorinity was exceptionally low (0.2‰ Cl in July 1972). In other years the mean chlorinity of all the water in the pool has usually been about 5‰ Cl, declining to about 2‰ in midsummer. Surface chlorinities vary far more widely than this (with varying weather conditions) but are rarely outside the range 0.3–4‰; the overall surface average is about 1.2‰. In summer the hypolimnion is anoxic and its pH is near 7, while the less saline upper layers are saturated with oxygen and their pH is frequently as high as 9. There are only small variations of temperature with depth, but in summer the pool temperature is often higher than the temperature of the inflowing fresh water by 5 °C. The factors governing surface-average chlorinity, fluctuations of chlorinity and the elimination of chloride from the pool are discussed. So long as the pool is stratified, wind speeds would appear to be as important as rainfall in determining the surface chlorinities and the rate of loss of chloride back to the sea. The distribution of some animals (such as Procerodes, Victorella and Eucrangonyx) are shown to be related to the salinity system in ways that strongly suggest a causal relationship, but for others (such as Gammarus chevreuxi and Plumatella) it is less easy to formulate plausible explanations at this stage. The pool is briefly compared with similar brackish lagoons elsewhere and its place in existing classifications of brackish waters is discussed.