Spatial distribution of epiphytic chironomid larvae in a shallow macrophyte-dominated lake: effect of macrophyte species and food resources

Four submerged macrophytes were analysed with their respective assemblages of chironomid and algal taxa. Studies were made bimonthly, from January to November, in a shallow macrophyte-dominated lake. The periphytic algal community was represented by chlorophytes, diatoms and cyanobacteria; diatoms showed the highest relative abundance on the macrophyte species during the study. Algal biomass was affected by the season, and in January the highest values were found on Stratiotes aloides L., Potamogeton lucens L., Myriophyllum spicatum L.; in July on Chara aculeolata Kütz. The lowest algal biomass, on most of the macrophytes studied, was found in September. Larvae of Dicrotendipes sp., Diplocladius cultriger Kieffer, Endochironomus albipennis gr., Endochironomus impar (Walker), Glyptotendipes sp. and Paratanytarsus austriacus Kieffer were dominant within the chironomid community. The spatial distribution of chironomids between macrophytes species was related to periphytic algae biomass and certain environmental variables (temperature, water transparency, biomass of chlorophytes and diatoms). Analysis of chironomid diet and the results of canonical correspondence analysis confirmed this relationship. The strongest chironomid-algae relationships were observed on S. aloides and P. lucens.     

Comparative feeding ecology of two herbivorous damselfishes (Pomacentridae: Teleostei) from the Gulf of California,Mexico

Two species of benthic damselfishes from the Gulf of California, Mexico, use contrasting behaviors when feeding on benthic algal communities. The small (±70 g) Cortez damselfish, Eupomacentrus rectifraenum (Gill, 1862), feeds selectively from a multi-species algal mat, eats fleshy red and green algae and ignores brown and calcareous algae. The giant blue damselfish, Microspathodon dorsalis (Gill, 1862), is a large (±450 g), lethargic, nonselective feeder which grazes on a near monoculture of a fleshy red alga, Polysiphonia sp. Feeding activity for both species is low in the morning peaks during late afternoon, and drops sharply as night approaches. Based on feeding rates, gut-filling times, and weights of gut contents, Cortez damselfish process six to eight full guts of food and giant blue damselfish three full guts of food per day. The algal mat exhibits high standing crops (291–618 g dry wt · m^?2) and low productivity, but the preferred food of the Cortez damselfish (Ulva) appears to colonize the mat frequently and grow rapidly. The Polysiphonia dominated community on giant blue damselfish territories exhibits low standing crops (23 g · m^?2) and high productivity (34–47 times that of the mat per gram algae). Even though the feeding behaviors and resources used by the two damselfishes differ, both species eat similar food (delicate red and green fleshy algae, and depend on rapid colonization and/or high productivity to maintain their primary foods in the grazed algal community.     

The impact of small chironomid grazers on epiphytic algal abundance and dispersion

• 1 A combination of field and laboratory experiments was used to assess the impact of chironomid grazers on taxonomic composition, abundance and dispersion of epiphytic algal assemblages.
• 2 In the laboratory, Psectrodadius sp. reduced the biovolume of algal species preferred as food and increased the degree of clumping of non-preferred species. Thienemanniella cf. fusca had both positive and negative effects (depending on the algal species) on the biovolumes of algal species preferred as food and increased the degree of clumping of non-preferred species.
• 3 In field exclosures, no effect of removal of chironomid larvae from the grazer assemblage could be detected in autumn or winter experiments. A third, longer removal experiment, conducted in summer, resulted in increased biovolumes of edible Cosmarium spp. and Aphanocapsa spp., preferred foods of chironomid larvae. Biovolumes of Lyngbya sp., Eulbochaete spp. and Oedogortium spp., filamentous taxa used extensively in larval case construction, also increased. Chironomid larvae had no effect on total algal biovolume or biovolume of large unicellular algae.
• 4 Chironomid larvae can influence epiphytic algal assemblages through selective grazing by reducing the biovolumes of preferred foods and through case-building activity by reducing the biovolumes of construction materials.

     

Green urchin as a significant source of fecal particulate organic matter within nearshore benthic ecosystems

The role of green sea urchin Strongylocentrotus droebachiensis as a source of fecal particulate organic matter (POM) for the benthic nearshore ecosystems has been studied over a 3.5-month period. Three macroalgae were tested as food sources: Alaria esculenta, Laminaria longicruris and Ulvaria obscura. Urchins were fed ad libitum with either a single alga species or a mixture of all three algae. Consumption and defecation rates were determined as well as the feces/alga ratio in term of biomass and biochemical composition. Consumption rate increased exponentially with urchin size and also varied with alga species. In the single alga trial, consumption rate was higher for both brown algae (Laminaria and Alaria) compared to Ulvaria. Urchins feeding on the mixture of algae maintained their total ingestion rate (sum of the three algae) at the same level to those feeding on a single alga diet. The mixed algae trial showed that urchins clearly preferred Laminaria (72% of total ingestion) over Alaria (22%) and Ulvaria (6%). The defecation rate was tightly correlated with the food consumption rate and thus increased with urchin size. On average, 75% of the ingested algal biomass was released as fecal POM. The percentage of food defecated changed with alga species, with the highest value for Alaria (81%) and the lowest for Laminaria (67%). The percentage of food defecated by urchins feeding on the mixture of algae was generally comparable to those feeding on single alga diet. Biochemical composition (in soluble carbohydrates, proteins and lipids) of urchin fecal POM reflected that of the algae content. From 40% to 80% of macronutrients in algal food persisted in fecal matter. This proportion varied with the alga species and macronutrient considered. This study shows that the green sea urchin plays a significant role in the production of POM within nearshore benthic ecosystems, and it is a potentially nutritious food source for detritivores.     

Development of filamentous green algae in the benthic algal community in a littoral sand-beach zone of Lake Biwa

Temporal changes of biomass and dominant species in benthic algal communities were investigated in a littoral sand-beach zone in the north basin of Lake Biwa from December 1999 to September 2000. Chlorophyll-a amounts of benthic algal communities per unit area of the sandy sediments rapidly increased from late April to June. Increases in biomass of the benthic algal communities are considered to result from the propagation of filamentous green algae Oedogonium sp. and Spirogyra sp. The cell numbers of filamentous green algae and chlorophyll-a amounts of benthic algal communities at depths of 30 and 50cm at a station protected by a breakwater in May were significantly higher than those of a station exposed directly to wave activity. Thus, the biomass accumulation of the benthic algal communities seems to be regulated strongly by wave disturbance. The development of filamentous green algae may contribute to the increase in biomass of the benthic algal community and to the changes in seasonal patterns of biomass in the sand-beach zone of Lake Biwa. We consider that the development of the filamentous green algal community in the littoral zone of Lake Biwa is the result of eutrophication.     

Historical changes and inventory of macroalgae from Königshafen Bay in the northern Wadden Sea

Knowledge on the distribution, abundance and species richness of intertidal macroalgae occurring on sandy and muddy flats of the German Wadden Sea is still incomplete. We summarize published and unpublished information available on the presence of macroalgae on the tidal flats of Königshafen Bay (island of Sylt, North Sea), one of the more extensively studied areas of the Wadden Sea. A total of 46 green algal species, 36 brown algal species and 26 red algal species has been recorded within the last 120 years on soft and hard substrata of Königshafen Bay (disregarding species found unattached or drifting). Several of these species were only temporarily resident on the tidal flats. Today, at least 35 green, 15 brown and 12 red algal species occur within or close to Königshafen Bay. Significant long-term changes in species abundances have occurred in all three major groups of algae: Since the late 1970s, dense green algal mats dominated byEnteromorpha flexuosa, E. radiata andE. prolifera have occurred regularly on the intertidal flats, whereas a general decrease of brown and red algae has been documented. Two red algal species,Gracilaria verrucosa and its epiphyteCallithamnion corymbosum, were conspicuous members of the macroflora until the middle of this century. Although still present in the 1980s, they have now disappeared completely. On the other hand, the brown algaSargassum muticum has begun to colonize mussel beds. The causal background of long-term changes in the macroalgal flora of Königshafen Bay is discussed. Owing to substantial nomenclatural changes during the last 120 years, a revised species list with authors’ names and synonyms is included.     

Seasonal dynamics and spatial distribution of epiphytic dinoflagellates in Peter the Great Bay (Sea of Japan) with special emphasis on Ostreopsis species

Studies of epiphytic dinoflagellates in Peter the Great Bay, Sea of Japan in 2008–2011 revealed the presence of 13 species. Five of the species are known as potentially toxic: Amphidinium carterae, A. operculatum, Ostreopsis cf. ovata, O. cf. siamensis and Prorocentrum lima. The maximum species richness and abundance of epiphytic dinoflagellates were observed in autumn (from September to October). Ostreopsis spp. were most widely distributed and predominated, amounting to 99% of the total density of dinoflagellates. Multi-year seasonal dynamics of Ostreopsis spp. in Peter the Great Bay showed that these cells appear as epiphyton in August after maximum warming of surface waters (22–24 °С) and disappear in early November, when the water temperature decreases below 7 °С. Ostreopsis spp. proliferation occurred in September, when the water temperature was 17.2–21.0 °C. The highest densities of Ostreopsis spp. were recorded on September 9, 2010 on the rhodophyte Neorhodomela aculeata – 230 × 10³ cells g⁻¹ DW or 52 × 10³ cells g⁻¹ FW. The spatial distribution of epiphytic dinoflagellates was investigated in the near-shore areas of Peter the Great Bay during the second half of September 2010 to evaluate the role of hydrodynamic conditions. Epiphytic dinoflagellates were not found in sheltered sites having weak mixing hydrodynamics. However, the abundances of Ostreopsis spp. were significantly higher at sites having moderate turbulence compared to biotopes experiencing strong wave action. Densities of Ostreopsis spp. were not significantly different on macrophytes with branched thallus of all taxonomic divisions. However, the average cell densities of Ostreopsis spp. on green algae with branched thallus were significantly higher than on green algae having laminar thallus.     

宁夏沙坡头地区藻类及其分布

通过野外采样,室内培养、观察、鉴定,研究了沙坡头地区藻类多样性和分布特点,并为当地藻类资源利用提出建议。研究发现沙坡头地区藻类植物共计40种(包括1变型),其中蓝藻17种,绿藻10种,硅藻9种,裸藻4种,全为普生种,陆地生境中种类最丰富。亚气及陆地生境中蓝藻在种类数(除灌溉林地结皮)、生物量方面占主导地位;贫瘠水体生境中硅藻占主要地位,营养较丰富水体中绿藻处于优势地位。另外,陆生生境中多以具鞘微鞘藻Microcoleus vaginatus为优势种,小席藻Phormidium tenue为主要种。     

Marine macroalgae as foods for fishes: an evaluation of potential food quality

Synopsis A revitalized view of feeding by herbivorous marine fishes is sought through two questions. First, What characteristics of major taxa of algae identify them as predictably high or low quality foods? Second, are marine algae valuable foods for fishes which do not mechanically disrupt cell walls and do not harbor specialized enzymes or microbes capable of lysing cell walls? Energy, ash and nutrient content of 16 species of marine algae were employed to assess food quality of fleshy red, green, brown and calcareous red algae. On the basis of ash, calories, total protein and total lipid content, fleshy algae should be superior to calcareous algae as foods for fishes; in addition, green algae should be superior to brown algae and brown algae superior to red algae. When the probable digestibility of storage and extracellular carbohydrates is considered, green and red algae are predicted superior to brown algae as food. Two species of damselfishes (Pomacentridae) from the Gulf of California,Eupomacentrus rectifraenum andMicrospathodon dorsalis, eat red and green algae and ignore brown and calcareous algae. They feed, therefore, in a fashion consistent with predictions based only on algal chemistry. These fishes absorb at least 20–24% of the biomass, 57–67% of the protein, 46–56% of the lipid and 37–44% of the carbohydrate contained in algae eaten in the wild. Since these damselfishes do not masticate their food, it appears that herbivorous fishes can digest major fractions of algal nutrients without mechanical destruction of algal cells.     

The stable isotope of nitrogen in an experimental culture of Ulva spp. and its assimilation in the nutrition of white shrimp Litopenaeus vannamei, Baja California Sur, Mexico

Stable nitrogen isotope ratios have been used to study the incorporation of nitrogen into the food webs of marine systems. Some species of algae can be cocultured with shrimp, resulting in a sustainable alternative to reduce or eliminate the use of commercial food. One option is the development of Ulva spp. in open-air ponds under a rigorous control of water quality. Recently, the coculture of Ulva spp. and juvenile shrimp (in aquaria and open-air ponds) has shown, under stereomicroscope observation, that the crustaceans were feeding on the Ulva spp. The consumption of commercial food and Ulva spp. by juvenile shrimp has been evaluated to establish the uptake of nitrogen into tissues of this crustacean. The muscle tissue of juvenile shrimp initially assimilated nitrogen from commercial feed and later the cocultured shrimp assimilated nitrogen from the Ulva spp., which demonstrated the potential application of live and fresh diets and the optimization of their use in diets containing very low levels or no commercial food.     

Factors influencing algal consumption and feeding rate in Heterotrissocladius changi Saether and Polypedilum nebeculosum (Meigen) (Chironomidae: Diptera)

Summary Factors influencing the consumption of algae and the rate of feeding by the larvae of Heterotrissocladius changi Saether and Polypedilum nebeculosum (Meigen) (Chironomidae: Diptera) were determined from collections made between July 1975 and May 1977 in Yellowknife Bay, situated in the Canadian subarctic. Although both species fed heavily on algae during the summer, few cells were ingested in the winter, coincident with an overall reduction in feeding intensity. H. changi fed primarily on benthic species (Achnanthes minutissima, Achnanthes pinnata, Fragilaria pinnata, Fragilaria vaucheriae, Navicula spp., Scenedesmus spp.) Whereas P. nebeculosum was restricted to planktonic forms (Dinobryon spp., Scenedesmus spp.). The consumption of algae by H. changi depended largely on the density of the microflora in the environment. This factor had little influence on P. nebeculosum and was replaced by size selection, which prevented the ingestion of many planktonic algal species. The importance of algae to both chironomids also depended on the susceptability of certain algal forms to digestion. Temperature generally regulated the intensity of feeding of both H. changi and P. nebeculosum. However, changing day-length initiated heavy feeding during April and May, despite low water temperatures. The depth of water and the organic content of the substrate had no detectable effect on the amount of material in the guts.     

How protective are refuges? Quantifying algal protection in rock crevices

1. Refuges can be functionally important if they harbour sufficient organisms during disturbance to augment population recovery. I quantified the protection of stream algae in crevice refuges using the applied, severe disturbance of scrubbing. Scrubbing effectively removed visible surface algae, and algae remaining on stones were considered protected. 2. In a field experiment, substrata with different quantities of crevice (glass bottles; greywacke; schist; pumice) were incubated in a channelised stream. The possible influence of growth conditions was investigated concurrently; half of the substrata were suspended in the water column, the rest were placed on the bottom (providing differential access to grazers and exposing them to different flow conditions). 3. Rougher substrata had greater total algal biomass than smoother substrata; this pattern resulted from more algal biomass in crevices of the rougher substrata. Protection from scrubbing ranged from about 5% of total algal biomass on glass and greywacke to 80% on pumice. In contrast, algal biomass removed by scrubbing was similar among the experimental substrata. Suspended substrata had more chironomid grazers than those on the bottom, and also greater algal biomass, possibly because of high algal concentrations in the chironomid retreats. 4. A survey of stones from three rivers supported the experimental results; namely, rough pumice protected more algae from scrubbing than did smoother greywacke. 5. In a separate experimental assay, there was no difference in algal growth on agar plates with and without added powdered rock substrata, suggesting that crevice characteristics and not substratum chemistry produced the differences in algal assemblages between rough and smooth surfaces. 6. Results indicate that rough stream stones may protect sufficient algae to augment their recovery in streams following disturbance.     

Carbon sources for lake food webs in the Canadian High Arctic and other regions of Arctic North America

We investigated the role of autochthonous and terrestrial carbon in supporting aquatic food webs in the Canadian High Arctic by determining the diet of the dominant primary consumer, aquatic chironomids. These organisms were studied in fresh waters on 3 islands of the Arctic Archipelago (~74–76°N) including barren polar desert watersheds and a polar oasis with lush meadows. Stomach content analysis of 578 larvae indicated that chironomids primarily ingested diatoms and sediment detritus with little variation among most genera. Carbon and nitrogen stable isotope mixing models applied to 2 lakes indicated that benthic algae contributed 68–95% to chironomid diet at a polar desert site and 70–78% at a polar oasis site. Detritus, originating from either phytoplankton or terrestrial sources, also contributed minor amounts to chironomid diet (0–32%). Radiocarbon measurements for the 2 lakes showed that old terrestrial carbon did not support chironomid production. Carbon stable isotope ratios of chironomids in other High Arctic lakes provided further dietary evidence that was consistent with mixing model results. These findings indicate that, in the Canadian High Arctic, chironomids (and fish that consume them) are supported primarily by benthic algae in both polar desert and oasis lakes. In contrast, our review of carbon flow studies for lakes in other Arctic regions of North America shows that terrestrial carbon and phytoplankton can be important energy sources for consumers. This study provides a baseline to detect future climate-related impacts on carbon pathways in High Arctic lakes.     

A comparison of the trophic ecology of the crayfishes (Orconectes nais (Faxon) and Orconectes neglectus (Faxon)) and the central stoneroller minnow ( Campostoma anomalum (Rafinesque)): omnivory in a tallgrass prairie stream

Omnivorous fish, such as the central stoneroller minnow (Campostoma anomalum(Rafinesque)), and crayfish often play important roles in the trophic dynamics of streams. The trophic role of these two omnivores has not been compared within a system even though they both consume algae, detritus and invertebrates and often co-occur in streams in the Midwestern United States. Natural abundance of ¹⁵N and ¹³C isotopes and a whole stream ¹⁵N-labeled ammonium chloride release were used to compare the trophic ecology of the central stoneroller minnow (Campostoma anomalum (Rafinesque)) and two species of crayfish (Orconectes neglectus (Faxon) and Orconectes nais (Faxon)) in a tallgrass prairie stream. The ¹⁵N and ¹³C values of Orconectes spp. were more similar to coarse benthic organic matter (CBOM) and filamentous green algae than to invertebrates, fine benthic organic matter (FBOM), and periphyton. Values for ¹⁵N and ¹³C in C. anomalum were more similar to grazer and collector invertebrates and filamentous green algae than to FBOM and periphyton. Results from a ¹⁵N tracer release also indicated a portion of algae and/or invertebrates were a component of nitrogen assimilated in Orconectes spp. and C. anomalum diets. Gut contents of C. anomalum were also analyzed. In contrast to stable isotope data, amorphous detritus was a significant component of C. anomalum guts, followed by diatoms and filamentous green algae. A significant percentage of invertebrate material was found in C. anomalum guts sampled in the spring. Experiments were conducted in artificial streams to determine if Orconectes spp. and C. anomalum could reduce epilithic algal biomass in small streams. Algal biomass on clay tile substrata was decreased relative to controls in artificial stream channels containing O. neglectus (3.4 fold, p=0.0002), C. anomalum (2.1 fold, p=0.0012), and both species combined (3.0 fold, p=0.0003). Results indicate that Orconectes spp. are functioning more as algal and detrital processors than as predators in Kings Creek. Isotope and gut content data show that C. anomalum includes invertebrates as well as algae and detritus in its diet. Both species have the potential to affect algal biomass and are important omnivores in the stream food web.     

Territorial behavior of threespot damselfish (Eupomacentrus planifrons) increases reef algal biomass and productivity

Synopsis Algal growth and damselfish (Eupomacentrus planifrons) territories were studied in two reef habitats at Discovery Bay, Jamaica. Damselfish territories were contiguous in the reef flat (0 to 2.5 m), where the algal composition and biomass varied from territory to territory. In contrast, on the lower reef terrace (22 m), damselfish territories were often spatially segregated. While the algal composition of the territories was more uniform on the reef terrace, the total algal biomass was lower than in the territories on the reef flat. Damselfish are largely herbivorous, and they defend their territories against most intruding fish, including a number of herbivorous species. Areas of the reef terrace outside of damselfish territories were heavily grazed by herbivorous fishes and contained only small quantities of non-crustose algae.The reef terrace territories were characterized by a multispecific turf of algae (greens, blue-greens, and reds) covering the Acropora cervicornis framework and by the leafy, brown alga, Lobophora variegata. A rapid reduction in the biomass of brown algae and filamentous algae was noted when damselfish were permanently removed from their territories. Only calcified, encrusting algae — plants apparently somewhat undesirable as fish food sources — would be common on the terrace zone of this reef if damselfish territories were absent. Damselfish territoriality may significantly influence the dynamics of some reefs by increasing the biomass of the algal turf thereby increasing; reef productivity. Since blue-green algae, potential nitrogen fixers, occur in these algal turfs, the fish may also be indirectly affecting reef nutrition.     

Competition and succession between the oily alga Botryococcus braunii and two green algae Chlorella vulgaris and Chlamydomonas reinhardtii

Contamination by other algal species is a major problem in the culture of the oil-producing green alga Botryococcus braunii (Bb). In this study, the population interactions between Bb and two small green algae, Chlorella vulgaris (Cv) and Chlamydomonas reinhardtii (Cr), were separately investigated in two controlled experiments. Filtrates of Cv or Cr significantly stimulated the growth of Bb in the first 9–11 days when the filtrates were derived from a low- or medium-density algal culture. However, filtrates of Cv and Cr from the high-cell-density cultures significantly inhibited Bb growth from day 11 and 7, respectively. In Bb–Cv competition, the growth of Bb was significantly inhibited by Cv at the biomass ratio of Bb/Cv?=?1:1. However, when the Cv component reduced to 0.5 (Bb/Cv?=?1:0.5) or 0.25 (Bb/Cv?=?1:0.25), the growth of Bb was faster than that in the control in the first 13 days. Similarly, in Bb–Cr competition, the biomass ratios of 1:0.5 and 1:0.25 (Bb/Cr) favoured the growth of B. braunii in the first 15 days, but the growth of Bb decreased in the Bb/Cr ratio of 1:1 from day 15. By contrast, the growth of Cv and Cr were not affected by the presence of Bb in all biomass ratios. These results suggest that Cv and Cr could impact the growth of Bb by metabolites in the culture medium, and when the biomass of Bb is two to four times that of other algae, this oily alga becomes competitive over other algal species. This study reveals the mechanism of population dynamics of this oily alga competing with small and fast-growing algae. The results are applicable in reducing algal competition and regulating species succession between slow and fast-growing algae species.     

Food habits, feeding behaviour and impact of triploid bighead carp, Hypophthalmichthys nobilis, in experimental ponds

Food of sterile triploid bighead carp, Hypophthalmichthys nobilis , was examined in ponds receiving water from a hypereutrophic take in Florida. No distinctive seasonal changes in food composition were found. The fish fed selectively on Botryococcus braunii Kuetzing, a large nuisance algal species, which constituted 61% of the volume and 50% of the dry weight of the food. Zooplankton was highly selected, but constituted only 3% of the food volume because of the low zooplankton density in the ponds. The remaining proportion of food consisted mainly of blue-green algae. Final fish biomass was low, ranging from 60 to 97 kg ha⁻¹ in individual ponds. Low biomass was due to slow fish growth and high fish mortality. Despite low biomass, the fish lowered the ratio of blue-green/green algae in the ponds and tended to lower phyto- and zooplankton abundance.     

Biomass, species composition and diversity of epipelic algae in mire pools

The biomass, species composition and diversity of epipelic algae in two small pools of contrasting physicochemical characteristics in Miyatoko Mire were studied in 1992 (pool 3 =site B4 and pool 50 =site D2). A total of 93 species and 67 species of epipelic algae occurred at sites B4 and D2, respectively. Considerable differences were observed between the two sites in the seasonal fluctuations of species number, biomass and dominant species. At site B4, little changed with species number during April–August and markedly increased in October, while biomass was largest in April and gradually decreased during June–October. Diatoms and desmids occupied 33–82% and 15–63% of total algal biomass, respectively. At site D2, species number and biomass were small in April just after the snow-thaw, and increased in June and decreased in August and October. Diatoms occupied 90–98% of total algal biomass. The species diversity was always higher at site D2 than B4. As a result of analyses of water chemistry in the two pools, pool B4 can be recognized as a habitat experiencing high disturbance frequency. It is predicted that pools experiencing frequent disturbance will have less epipelic algal biomass and diversity.     

A primary study on relationships between subfossil chironomids and the distribution of aquatic macrophytes in three lowland floodplain lakes,China

Aquatic macrophytes act as an important substrate for zoobenthos-supporting midge communities (chironomids) that can be used for reconstructing past environmental change. This study selected three macrophyte-dominated lakes in the middle reach of the Yangtze floodplain (central China), to identify relationships between macrophyte characteristics (i.e. communities, richness and biomass) and subfossil chironomid assemblages. One-way ANOSIM tests illustrated that there were significant differences in chironomid community compositions between lakes. Most chironomid taxa were not selective of dominant macrophyte species, but Corynoneura species were found to be associated with the development of Vallisneria as revealed by SIMPER analysis. Multivariate analyses indicate that submerged plant biomass, water depth and secchi depth, conductivity and the concentration of HCO₃ ^? were significantly correlated with midge compositions. Overall, this study corroborates the existing opinions that chironomid species show little selectivity to plant type but are significantly influenced by plant density. As an exception to this general trend, we found strong associations between Corynoneura chironomid larvae and Vallisneria macrophytes, prompting the need for future experimental studies to confirm this association.     

Seasonal patterns of food availability: influences on the reproductive output and body condition of the herbivorous crab Grapsus albolineatus

 Rocky shores in Hong Kong experience marked seasonal differences in climate resulting in seasonal changes in macroalgal assemblages. The tropical rocky shore crab, Grapsus albolineatus, feeds selectively on filamentous algae through the year but the abundance of these algae and foliose algae is greatly reduced during the summer when encrusting algae dominate the shores and the crab’s diet. This switch in diet may have implications for the reproductive output of this crab. Standing crop of algae varied greatly through the year, peaking in March and April, when the nutritional quality of the algae was also highest. On the shore, available algal protein and energy were both lowest in July. The crab selected an algal diet rich in nutrients than that available to it on the shore for all months of the year except September to December for protein, and July and August for energy. The input of animal matter considerably increased the protein content of the diet, but made little difference to the energy content. Growth and body condition were greatest during March and June, coinciding with the peak in algal biomass. Storage of nutrients in the hepatopancreas of G. albolineatus commenced in November, coinciding with the increase in biomass and quality of algae on the shores, and then, peaking in May and June, these nutrients were utilised by the reproductive organs during the reproductive season which ran from April to November. The middle of the reproductive season coincided with the period when the standing crop of algae on the shore was at its lowest levels and poorest quality. Cycles of growth, reproduction and storage in G. albolineatus appear to be directly influenced by seasonal patterns of algal food availability. Nutrient storage in the hepatopancreas, during periods when food is nutritionally rich and is most abundant, does not guarantee reproductive success of the crab, but appears to be a prerequisite. Selection of an optimal diet (energy- and protein-rich) in a seasonal environment by ingesting abundant nutrient-rich algal species and the opportunistic consumption of animal matter strongly influences growth and reproductive output of G. albolineatus. Received: 18 March 1996 / Accepted: 17 July 1996