Biomass, diversity and production of rocky shore macroalgae at two nutrient enrichment and wave action levels

The littoral zone of temperate rocky shores is normally dominated by perennial macroalgae (e.g. Fucus, Ascophyllum, Laminaria), but nutrient enrichment and/or permanently decreased wave action may lead to structural community changes from dominance of perennials to increased amounts of annual opportunistic species (mainly green algae). Macroalgal biomass, diversity and production as well as relationships between the two latter were studied using Solbergstrand’s rocky shore mesocosms in SE Norway in connection with a long-term experimental manipulation of nutrient addition and wave action (high and low levels of both factors applied in a crossed way to eight outdoor basins). After more than 2 years of experimental treatment, the total standing stock of macroalgae was larger in low nutrient than in high nutrient treatments as well as in high wave compared to low wave treatments (in autumn only). For macroalgal functional groups, bushy and filamentous brown and filamentous red algae were generally favoured by low nutrient concentrations, while annual filamentous and sheet-like green algae were stimulated by the nutrient enrichment. There was only one significant interaction between nutrient enrichment and wave action (for brown filamentous algae in autumn) and also only one significant main effect of the wave treatment (for bushy brown algae in autumn). Surprisingly, the high nutrient treatments supported a higher diversity of macroalgae, whereas the low nutrient treatments generally showed higher production rates. Moreover, significantly negative correlations were found between macroalgal diversity and primary productivity in both summer and autumn. This study shows that it is the biological components of the communities subjected to external forcing (nutrient addition or decreased wave action) that regulate production and this contradicts the common misperception that resource production in natural systems simply can be fast-forwarded by fertilization. The negative relationships between diversity and productivity, although a consequence of unexpected results for diversity and production, are also novel and hint towards species identities having more important functional consequences than general species dominance patterns and the amount of species per se. These results also emphasise the context dependency of findings within the field of biodiversity and ecosystem functioning.     

High natural mortality of Baltic herring (Clupea harengus) eggs caused by algal exudates?

Results from field studies in 1978 and 1982 together with results from a laboratory study carried out in 1984 indicate that the unusually high natural mortality of Baltic herring (Clupea harengus L.) eggs observed in situ is presumably caused by toxic exudates released by filamentous brown algae during a limited period which happens to coincide with the peak of spawning. Oxygen deficiency is no longer believed to be the major cause of the observed unnaturally high mortality. It cannot, however, be entirely ruled out.     

Effects of increased vegetation cover on nesting behavior of sticklebacks (Gasterosteus aculeatus)

To maximize reproductive success, reproductive behaviors have to be adjusted to environmental conditions. The breeding habitats of the threespine stickleback Gasterosteus aculeatus are presently changing in the Baltic Sea due to eutrophication and enhanced growth of filamentous green algae. Earlier studies show that high algae density reduces mate encounter rate and the intensity of selection on a few sexually selected traits, but the willingness of males to nest under the new conditions, and their investment into reproduction, have not been investigated. Here, we investigated the effect that increased algae growth has on nesting behavior of sticklebacks. We allowed males to build nests in aquaria with either a sparse or a dense vegetation structure and recorded male nest building behavior and nest characters. Males that nested in dense vegetation showed a longer latency before commencing nest building and also completed nest building later since there was no difference in time spent building. There was no effect of vegetation cover on nest characters. These results suggest that males prefer to nest in sparser vegetation, but that they eventually nest in dense vegetation if more open habitats are not available. This may arise from a negative effect of vegetation on visibility and mate encounter rate and thus on expected reproductive success. This implies that human-induced changes in the environment may force sticklebacks to breed in non-preferred habitats. Since the intensity of sexual selection is reduced in these areas, this could influence the further evolution of the population.     

Selective consumption and facilitation by mesograzers in adult and colonizing macroalgal assemblages

Grazing pressure on macroalgae in littoral communities may vary with algal species, the age of an algal individual and grazer identity. Previous studies on alga–grazer interactions have shown that grazer preference for an algal species may release another one from interspecific competition. We measured the impacts of four common grazer taxa and the natural grazer guild on macroalgal communities at both their colonization and adult stages, and compared the impacts to grazer exclosures. The grazer effects were stronger on colonizing than on adult macroalgae; grazers did not reduce the total density of adult algae. Grazers both feed on propagules and indirectly facilitate other algae, depending on the grazer or algal species. Hydrobia species increased the settlement of spores of the red alga Ceramium tenuicorne. Similarly, the gastropod Theodoxus fluviatilis tended to facilitate one crustose algal species, but decreased the propagule density of annual filamentous algae, suggesting a preference for one species to the advantage of another. Effects of crustacean mesograzers on the studied macroalgae were weak. These results indicate that northern Baltic macroalgae are limited to grazing mainly during their colonization stage.     

Effect of phosphate and ammonium levels on photosynthetic and respiratory responses of the red alga Gracilaria verrucosa

Gracilaria verrucosa (Hudson) Papenfuss exposed to nutrient enriched media (0.1 mM PO₄; 1.0 mM NH ₄ ⁺ ) by pulse feeding 2 h every third day for a period of 5 wk at 20°C and 25–30 salinity showed significantly higher rates of photosynthesis regardless of photon flux density correlated with increased pigment levels. Algae in nonenriched media showed significantly higher levels of soluble carbohydrates and decreased levels of phycoerythrin and chlorophyll a. Photosynthetic and respiratory responses to temperature 15°, 25°, 30°C and salinity (15, 25, 30 S) combinations indicate broad tolerances by both nutrient enriched and non-nutrient enriched algae. Photosynthetic and respiratory rates were highest at the high temperatures. Pulse-fed algae had significantly higher photosynthetic rates than non-nutrient enriched plants at all temperature and salinity combinations. Non-nutrient enriched algae had significantly higher respiratory rates than nutrient enriched algae at only 30°C and 15. The respiratory rates of both nutrient enriched and non-nutrient algae decreased under combinations of higher temperatures and salinities. G. verrucosa, grown without nutrients, has lower tolerances to environmental stresses.     

Mechanisms of copper tolerance in the marine fouling alga Ectocarpus siliculosus — Evidence for an exclusion mechanism

Copper-tolerant strains of the marine fouling alga Ectocarpus siliculosus (Dillw) Lyngb. have been recorded. The mechanisms of tolerance which may operate in a copper-tolerant strain were investigated by comparing the growth characteristics, copper content and extracellular organic production of tolerant and non-tolerant strains. Growth was measured by a wet-weight method. The copper content of dryashed cells was measured spectrophotometrically using tetraethylthiuram disulphide. The extracellular material is ultra-violet absorbing, and the production was measured by recording absorption at 265 nm. Evidence suggests that an exclusion mechanism is operating in the tolerant strain under investigation. The extracellular material, the cell wall or an associated organism make little or no contribution no this exclusion. Membrane and intracellular changes are believed to account for the tolerance in this strain of E. siliculosus.     

The role of the amphipod <Emphasis Type="Italic">Gammarus locusta</Emphasis> as a grazer on macroalgae in Swedish seagrass meadows

Mesograzers are thought to play a critical role in seagrass beds by preventing overgrowth of ephemeral algae. On the Swedish west coast, eelgrass Zostera marina has decreased in recent decades as a result of eutrophication and increased growth of macroalgal mats (mainly filamentous Ulva spp. and Ectocarpales), with no indication of grazer control of the algae. The aim of this study was to investigate the ability of the amphipod Gammarus locusta to control algal blooms during nutrient-enriched and ambient conditions, using a combination of laboratory, field and model studies. Laboratory experiments demonstrated that juvenile and adult G. locusta could consume both Ulva spp. and Ectocarpales, but that consumption of Ulva spp. was significantly higher. Cannibalism was common in individual treatments involving multiple size-classes of G. locusta, but only large, male gammarids consumed smaller juveniles in the presence of Ulva spp. as an alternative food source. However, no negative effects of cannibalism were found on total grazing impact. A model using size-specific grazing rates and growth rates of G. locusta and of Ulva spp. suggests that approximately 62 young juvenile, or 27 adult G. locusta are needed per gram DW of Ulva spp. to control the algal growth during ambient nutrient conditions, and approximately 2.6 times as many gammarids during enhanced nutrient conditions. On the Swedish west coast, densities and mean sizes of G. locusta in eelgrass beds are below these critical values, suggesting that the gammarids will not be able to control the growth of the filamentous macroalgae. However, in the field cage experiment, immigration of juveniles and reproduction of encaged adult G. locusta resulted in unexpectedly high densities of G. locusta (>4,000 individual m⁻²), and very low biomass of Ulva spp. in both ambient and nutrient-enriched treatments. Although the high numbers of juveniles in all cages precluded any significant treatment effects, this suggests that in the absent of predators, the population of G. locusta can grow significantly and control the biomass of Ulva spp. Furthermore, low grazing of Ectocarpales in the laboratory and high biomass of these filamentous brown algae in the field indicate a preference for the more palatable green algae Ulva spp. This study indicates that the high grazing capacity of G. locusta, in combination with high reproduction and growth rates, would allow the amphipod to play a key role in Z. marina ecosystems by controlling destructive blooms of filamentous green algae. However, high predation pressure appears to prevent large populations of G. locusta in eelgrass beds on the Swedish west coast today.     

Comparisons of nutrient uptake rates for Baltic macroalgae with different thallus morphologies

In-situ experiments were performed during different seasons to determine uptake rates of PO _3- ⁴ , NH ₊ ⁴ and NO _- ³ within ecologically representative ranges of nutrient concentrations, of dominant macroalgae in the Baltic Sea. Uptake rates were governed by nutrient concentrations, water temperature and thallus morphology, but not by the phylogenetic affinity of the species. Nitrogen uptake rates were always higher than those of phosphorus at the same concentrations, and NH ₊ ⁴ –N uptake rates exceeded those of NO _- ³ –N. The lowest uptake rates occurred among the late successional, long-lived, coarse species with low surface: volume ratios (Fucus vesiculosus, Furcellaria lumbricalis andPhyllophora truncata). The highest uptake rates were measured for short-lived, opportunistic algae, filamentous or with numerous hairs, (Cladophora glomerata, Enteromorpha ahlneriana, Scytosiphon lomentaria, Dictyosiphon foeniculaceus andCeramium tenuicorne). The latter group also had the highest V_max:k_max ratios, which indicates a more competitive advantage for nutrient uptake at low concentrations.     

Using opportunistic green macroalgae as indicators of nitrogen supply and sources to estuaries.

Nutrient inputs to estuaries are increasing worldwide, and anthropogenic contributions are increasingly complex and difficult to distinguish. Measurement of integrated effects of salinity and nutrient changes simultaneously can help ascertain whether N sources of similar magnitude and stable isotope (sigma15N) signatures are river dominated. We used Enteromorpha spp., an opportunistic macroalga, to obtain integrated measures of salinity, nutrient supply, and nutrient source in estuaries. We outplanted cultured algae in the field along spatial gradients within three southern California estuaries for 24 hours in wet and dry seasons. Tissue was analyzed for potassium (K+) to measure osmoregulatory changes, total nitrogen to examine changes in nutrient supply, and sigma15N to assess nutrient sources. Discrete measures of water salinity correlated with tissue K+ content; however, there was significant variability in the relationship, suggesting that the algae were subject to considerable variation in salinity over a tidal cycle. Tissue total N was not always related to snapshot measures of water column dissolved inorganic nitrogen (DIN), suggesting that integrated measures may be better at capturing the temporal and spatial complexity of nutrient availability. The combination of tissue K+, total N, and sigma15N measures revealed that inflowing rivers delivered N from watershed sources to Mugu Lagoon and Carpinteria Salt Marsh, while both the inflowing river and a mid estuary source were important sources of high sigma15N N in Upper Newport Bay. These experiments revealed complex patterns of supply and sources of N and demonstrate the usefulness of macroalgal indicators over water sampling to detect these patterns.     

Utilisation of mangroves and seagrasses by fishes in the Negombo Estuary,Sri Lanka

In two years of sampling, 10 287 fishes belonging to 62 species (39 families) were collected from mangroves and seagrasses of the Negombo Estuary. Of the species collected, 70% were marine, 27% estuarine and 3% freshwater. Aplocheilus melastigma, Ambassis dayi, Etroplus suratensis, E. maculatus, Bathygobius fuscus and Arothron immaculatus constituted 90% of the catch. Sixteen species were caught only from mangroves, 6 only from seagrasses, and 40 from both. The number of species, individuals, biomass and diversity were higher in seagrasses than in mangroves. Gobies and some siganids moved to the mangrove habitat as they grew, but cichlids, triacanthids, some eleotrids and tetraodontids preferred seagrasses. The number of species and individuals caught increased at the beginning of the north-east monsoon when the rainfall was low. The number of species and individual numbers were positively correlated with salinity and dissolved oxygen and negatively correlated with water temperature. Of the 33 species examined, seagrasses formed >25% of the gut content in juvenile Lates calcarifer, Apogon thermalis, E. suratensis, E. maculatus, Siganus javus, Butis butis and Bathygobius fuscus. Mangrove litter was the main gut content of the family Syngnathidae. E. suratensis, E. maculatus and S. javus from seagrasses contained more filamentous algae than seagrasses in their stomachs. S. javus collected from mangroves also contained more seagrasses in their stomachs than mangrove litter, indicating that the association of juvenile fishes with mangroves or seagrasses may not be obligate in estuaries subject to low tidal fluctuations.     

Reversed parasite-mediated selection in sticklebacks from eutrophied habitats

Sexual selection is often assumed to increase the viability of populations by increasing the quality of offspring produced. Presently, human-induced environmental changes are altering the process of sexual selection by influencing male–male interactions and female mate choice. Here, we show that increased density of filamentous algae due to eutrophication reverses parasite-mediated selection during reproduction in threespine sticklebacks (Gasterosteus aculeatus). When we manipulated the density of artificial algae in a breeding area in the Baltic Sea, more males nested in dense than in sparse vegetation, but the males in dense vegetation were more parasitized. Interestingly, heavily parasitized males acquired more eggs than less parasitized males in dense vegetation but not in sparse vegetation. The higher probability of reproduction for parasitized males in dense algae growth could be due to impaired visibility relaxing male–male competition or reducing female choosiness. This could favour males in poor condition that often invest more in attracting females than males in good condition. In sparse vegetation, parasitized males may have a lower reproductive success due to intense male–male competition, careful female choice and high predation rate selecting against parasitized males. The results suggest that eutrophication could alter the fraction of the population that reproduces, which could have long-term evolutionary consequences.     

Effects of water-body characteristics other than temperature and salinity on respiration in Euphausia pacifica

Ample evidence exists that many planktonic species are limited in their distribution to a given water mass. It has been suggested that the temperature-salinity values which define a given water mass are in themselves limiting these species' distribution. Other workers have suggested that other unique properties of a water mass may be involved. Euphausia pacifica is distributed throughout the Pacific subarctic water mass; its distribution also extends into British Columbia coastal waters. Experiments have shown that E. pacifica can withstand temperature changes of up to 15 C°; it can also withstand salinity changes of up to 10. Coastal E. pacifica appear to be able to better withstand stress from changes in temperature and salinity in water from some sources than in water from others. These differences are thought to result from differences in the past history of the water in question.Contribution No. 75008 from the Bigelow Laboratory for Ocean Sciences.     

Distribution of a marginal population of <Emphasis Type="Italic">Mytilus edulis</Emphasis>: responses to biotic and abiotic processes at different spatial scales

Physical and biological processes interact to produce pattern in nature. Pattern is scale dependent as processes generating pattern are heterogeneous in time and space. We tested some causes of variation in abundance and distribution of three marginal populations of sublittoral blue mussels, Mytilus edulis, in the non-tidal northeastern Baltic Sea. We studied the role of substrate inclination, perennial algae and siltation along local wave exposure gradients on mussel distribution over a regional salinity gradient. We found marked differences on regional scales (p < 0.001) with lower densities and biomasses of mussels with declining salinity. Along local gradients, mussel densities increased with increasing exposure (p < 0.001) and declining slope and sedimentation (p < 0.01). Site specifically, densities of blue mussels and the perennial red algae, Furcellaria lumbricalis, were positively related, results supported by a colonisation experiment. Also, young post-recruits showed significant relations to adult biomass, wave exposure, algal biomass, bottom slope and sediment cover. Findings showed that the relative importance of the determinants affecting blue mussels at the edge of their range vary with scale and are affected by the density and size structure of mussel populations. The study provides an indication of the types of factors that may be invoked as causes of spatial variation in marginal blue mussel populations and reinforces the need to consider multiple aspects when distributional patterns are assessed.     

The indirect effects of eutrophication on habitat choice and survival of fish larvae in the Baltic Sea

The structure of the habitat is usually crucial for growth and survival of young life stages. Presently, some nursery areas of fish larvae are changing due to eutrophication, e.g. due to enhanced growth of ephemeral filamentous algae at the expense of perennial species. We studied the influence of two habitats, one with filamentous algae (Cladophora glomerata) and the other with bladder wrack (Fucus vesiculosus), on habitat choice of pike larvae (Esox lucius) in the absence/presence of a predator or a competitor. We further tested whether the habitat choice is adaptive in increasing survival under predation threat. In contrast to expectations, pike larvae preferred the habitat with ephemeral filamentous algae to the bladder wrack, thriving in clean waters, independent of the presence/absence of both predator/competitor. In addition, the survival of the larvae was higher in the filamentous algae in the presence of predators, which suggested that the habitat preference of the larvae was adaptive. The structure of the bladderwrack habitat was probably too open for newly hatched larvae, which implies that F. vesiculosus and other large brown algae are not as important refuges for young larvae as previously thought.     

The effects of temperature and salinity on reproductive success of <Emphasis Type="Italic">Temora longicornis</Emphasis> in the Baltic Sea: a copepod coping with a tough situation

At specific locations within the Baltic Sea, thermoclines and haloclines can create rapid spatial and temporal changes in temperature (T) and salinity (S) exceeding 10°C and 9 psu with seasonal ranges in temperature exceeding 20°C. These wide ranges in abiotic factors affect the distribution and abundance of Baltic Sea copepods via species-specific, physiological-based impacts on vital rates. In this laboratory study, we characterized the influence of T and S on aspects of reproductive success and naupliar survival of a southwestern Baltic population of Temora longicornis (Copepoda: Calanoida). First, using ad libitum feeding conditions, we measured egg production (EP, no. of eggs female⁻¹ day⁻¹) at 12 different temperatures between 2.5 and 24°C, observing the highest mean EP at 16.9°C (12 eggs female⁻¹ day⁻¹). Next, the effect of S on EP and hatching success (HS, %) was quantified at 12°C for cohorts that had been acclimated to either 8, 14, 20 or 26 psu and tested at each of five salinities (8, 14, 20, 26 and 32 psu). The mean EP was highest for (and maximum EP similar among) 14, 20 and 26 psu cohorts when tested at their acclimation salinity whereas EP was lower at other salinities. For adults reared at 8 psu, a commonly encountered salinity in Baltic surface waters, EP was relatively low at all test salinities—a pattern indicative of osmotic stress. When incubated at 12°C and 15 different salinities between 0 and 34 psu, HS increased asymptotically with increasing S and was maximal (82.6–84.3%) between 24 and 26 psu. However, HS did depend upon the adult acclimation salinity. Finally, the 48-h survival of nauplii hatched and reared at 14 psu at one of six different temperatures (10, 12, 14, 16, 18 and 20°C) was measured after exposure to a novel salinity (either 7 or 20 psu). Upon exposure to 7 psu, 48-h naupliar mortality increased with increasing temperature, ranging from 26.7% at 10°C to 63.2% at 20°C. In contrast, after exposure to 20 psu, mortality was relatively low at all temperatures (1.7% at 10°C and ≤26.7% for all other temperatures). An intra-specific comparison of EP for three different T. longicornis populations revealed markedly different temperature optima and clearly demonstrated the negative impact of brackish (Baltic) salinities. Our results provide estimates of reproductive success and early survival of T. longicornis to the wide ranges of temperatures and salinities that will aid ongoing biophysical modeling examining climate impacts on this species within the Baltic Sea.     

Habitat structure and complexity as determinants of biodiversity in blue mussel beds on sublittoral rocky shores

Habitat-forming, ecosystem engineer species are common in most marine systems. Still, much uncertainty exists about how individual and population-level traits of these species contribute to ecosystem processes and how engineering species jointly affect biodiversity. In this manipulative field experiment, we examined how biodiversity in marginal blue mussel beds is affected by blue mussel (1) body size, density and patch context and (2) presence of fucoid and algal structures. In the study area, bladder-wrack (Fucus vesiculosus), filamentous algae and blue mussels (Mytilus edulis) coexist at shallow depths in a variety of patch configurations and offer complex habitats with a high variability of resources. We hypothesized that complexity in terms of mussel bed structure and algal presence determines species composition and abundance. Results from the experiment were compared with macrofaunal communities found in natural populations of both engineering species. Results show that the physical structure and blue mussel patch context are important determinants for species composition and abundance. Results further show that the presence of algal structures positively affects diversity in blue mussel habitats due to increased surface availability and complexity that these algae offer. This study shows that blue mussel beds at the very margin of their distribution have an indisputable function for promoting and maintaining biodiversity and suggest that facilitative effects of habitat-modifying species are important on Baltic Sea rocky shores with fundamental importance to community structure.     

Egg buoyancy of Baltic cod (Gadus morhua) and its implications for cod stock fluctuations in the Baltic

Successful spawning of Baltic cod (Gadus morhua L.) is restricted to the deep basins of the Baltic Sea and is dependent on abiotic conditions such as salinity and water oxygen content. Due to irregular inflows of saline water from the North Sea, the conditions for spawning fluctuate and consequently so does the cod stock. In May and June 1990 eggs were obtained from spawning cod caught off northern Gotland, Sweden. Our investigation revealed neutral egg buoyancy for Baltic cod at a salinity of 14.4±1.1 ppt, with a slight decrease some days before hatching. Today salinities of this magnitude occur only in the Bornholm basin, whereas in the Gdansk and Gotland basins the eggs sink toward the bottom and are exposed to lethal oxygen conditions.     

Comparative study on the population genetics of the red algae <Emphasis Type="Italic">Furcellaria lumbricalis</Emphasis> occupying different salinity conditions

Furcellaria lumbricalis is a red algae occurring in low salinity to fully marine conditions. Here, both putatively neutral and EST-derived microsatellite markers were developed and used to examine the genetic structure of northern European populations inhabiting different salinity conditions ranging from 35 to 3.6 psu. The amount of genetic variation did not differ between ocean and brackish populations, but differences were observed between marker types; EST-derived markers possessed less variation and showed greater differentiation than the putatively neutral microsatellites. No multicopy multilocus genotypes were detected despite expected asexuality in brackish populations. The Bayesian STRUCTURE analysis, when conducted for expressed marker data, indicated the presence of two main clusters, the Atlantic Ocean and Baltic Sea, while no clear structuring was observed based on putatively neutral microsatellites. The moderate level of genetic differentiation at neutral loci is probably due to genetic drift, a feasible explanation considering long distances between many populations, while the high level of differentiation in EST-linked markers reflects selection pressures.     

Photosynthetic rates of benthic marine algae in relation to light intensity and seasonal variations

The effect of light intensity on rate of photosynthesis was measured at irregular intervals over a 12-month period for 22 benthic marine algae in the western Baltic Sea. In most species photosynthesis (mg O₂·g dry weight^-1·h^-1) was highest in spring and summer, corresponding to the seasonal growth pattern of the algae. In winter all the species showed adaptation of the light compensation point. Highest productivity was shown by algae which are: short-lived annual species rather than perennials, eulittoral rather than sublittoral, and which possess sheet-like or filamentous thalli rather than coarsely branched forms. These factors are clearly inter-related.This research was carried out while one of us (R.J.K.) was an Alexander von Humboldt fellow at the University of Kiel, and is part of the programme Sonderforschungsbereich 95, Wechselwirkung Meer-Meeresboden, Universität Kiel.     

Chemoautotrophic symbiosis in the tropical clamSolemya occidentalis (Bivalvia: Protobranchia): ultrastructural and phylogenetic analysis

Patterns of algal seasonality, and their effect on the diet and feeding preferences of the herbivorous crab Grapsus albolineatus were investigated over an 18-mo period from March 1993 to August 1994 on an exposed tropical rocky shore (Hok Tsui Peninsula at Cape d' Aguilar, Hong Kong). Algal cover was greatest in the winter months, and lowest in the summer. Foliose algae such as Ulva fasciata, Porphyra suborbiculata, and Dermonema frappieri were dominant in the winter, but died off in the summer. During the hot summer months, perennial encrusting algae e.g., Ralfsia expansa, Hildenbrandia rubra, H. occidentalis, coralline crusts and the encrusting cyanobacteria Kyrtuthrix maculans, were the dominant algal species. Seasonal variation in algal abundance influenced the dietary selectivity of the herbivorous crab G. albolineatus. In the winter, the crab fed selectively on filamentous algae (e.g. Hincksia spp., Cladophora spp., Enteromorpha spp., and the cyanobacteria Lyngbya sp.). Foliose algae (e.g. U. fasciata, P. suborbiculata, Pterocladia tenuis) formed a small part of the diet, despite being the dominant species on the shore. Foliose and filamentous algae were virtually absent from the shore in the summer, and the crabs switched to feeding solely on encrusting algae. Electivity indices revealed preferences for green and brown turf species, and avoidance of foliose algae. Faecal analysis revealed that a greater proportion of the food is digested in the winter, suggesting that G. albolineatus is able to digest filamentous algae more efficiently than encrusting algae. Feeding preferences of G. albolineatus appear to be influenced by a number of factors, including the availability, digestibility and morphology of algae. The foraging behaviour and cheliped morphology of the crab also affect food choice. The monsoonal nature of Hong Kong's climate controls the diversity and abundance of intertidal algae and, therefore, indirectly influences the diet and subsequent growth and reproductive success of the herbivorous crab G. albolineatus.