Winter condition of marine plankton populations in Saanich Inlet,B.C., Canada. II. Micro-zooplankton

The abundance of micro-zooplankton (< 200 μm in size) was monitored weekly in Saanich Inlet throughout the winter, 1975–1976. Ciliate protozoans and metazoan nauplii were the major groups, with the former largely distributed in the top 5 m of the water column and the latter above 15 m. Average biomass throughout the winter was 3.1 mgC/m² for ciliates above 5 m and 5.5 mgC/m² for nauplii above 25 m, which made up 3.2 and 5.7% of the average phytoplankton biomass, respectively. They could consume over 30% of the phytoplankton production. Mesodinium sp. was an another abundant ciliate, but possibly a phyto-ciliate, with an average biomass of 8.2 mgC/m² above 5 m. Major energy flow at lower trophic levels during the winter was characterized by a ‘nanoflagellate → micro-zooplankton’ system which was comparable with the ‘centric diatoms → macro-zooplankton’ system occuring commonly at the other seasons in the inlet.     

EFFECTS OF SUBSTRATE RELIEF ON THE DISTRIBUTION OF PERIPHYTON IN LABORATORY STREAMS,I. HYDROLOGY1

We examined the hypothesis that the heterogeneity of epilithic algal assemblages in streams may be partly a result of hydrologic differences created when water flows over a rough substrate. A 32-day experiment was conducted in laboratory streams that contained either 22.5 × 22.5 × 4 cm or 7.5 × 22.5 × 4 cm tile blocks. Free water velocities in the streams overaged 28 cm·s^?1. Hydrologic parameters and algal assemblages associated with surfaces on top of blocks and with recessed surfaces between blocks were compared to corresponding surfaces in streams with of relief. In streams with blocks, shear velocities averaged 1.7 cm·s^?1 on the top of blocks and 0.8 cm·s^?1 in the recessed areas. Shear velocity at corresponding surfaces in the control (no relief) streams averaged 1.9 cm·s^?1 and exhibited little variation. The hydrologic differences created by the larger blocks significantly affected the distribution of algal biomass, with recessed areas having an average of 2.6 g·m^?2 AFDW more biomass than surfaces on the top of blocks. Differences in shear velocities and biomass accumulation between top and recessed areas for the smaller blocks were less than for large blocks. Successional changes on all substrates were similar with the exception that recessed surfaces had a significantly greater abundance of the filamentous chlorophyte Stigeoclonium tenue (Ag.) Kütz after day 16. The results suggest that in cobble riffle areas of natural streams, the interaction between current flow and substrate relief has the potential to create patches of algae which are different in biomass and taxonomic composition.     

河流鱼类分类群和功能群的纵向梯度格局——以新安江流域为例

确定鱼类群落的空间格局是保护和管理河流鱼类多样性的基础。尽管河流鱼类分类群（基于物种组成）的纵向梯度格局已得到大量报道,但其功能群（基于功能特征）的空间格局研究较少。以皖南山区新安江为研究流域,沿其"正源-下游"梯度共设置27个调查样点,分别于2017年5月和10月完成2次调查取样,着重研究了鱼类分类群和功能群结构的纵向梯度格局及其形成机制。共采集鱼类44种,可分为5个运动功能群和4个营养功能群,构成14个"营养-运动"复合功能群。双因素交互相似性分析结果显示,鱼类分类群和功能群均随河流级别显著变化,但两者均无显著的季节变化;根据相似性百分比分析,由1级至3级河流,数量优势物种和功能群的空间变化主要呈嵌套格局,而由3级至5级河流其变化主要呈周转格局。方差分解结果显示,局域栖息地、陆地景观和支流空间位置3类解释变量对分类群和功能群空间变化的解释率分别为33.6%和38.5%,其中,分类群受局域栖息地和支流空间位置变量的显著影响,而功能群受局域栖息地和陆地景观变量的显著影响。研究表明,沿着新安江的"上游-下游"纵向梯度,鱼类分类群和功能群的空间格局基本一致,但两者的形成机制不同：分类群的纵向梯度变化受环境过滤和扩散过程的联合影响,而功能群则主要受环境过滤影响。     

Relationships between fishes and habitat in rainforest streams in Sabah, Malaysia

Distinct fish assemblages were found at the mesohabitat scale in 14 streams in eastern Sabah, Malaysia. Sites were designated a priori as pool, run or riffle on the basis of physical habitat structure and properties. Principal components analysis of physical habitat data confirmed the validity of the a priori designation with a major axis of three correlated variables: water velocity, depth and substratum type. Canonical discriminant analysis on fish abundance and biomass data confirmed the existence of a specialized assemblage of fishes from riffle areas of all streams. Overall, pool and run assemblages were highly variable, dependent on stream size, but also variable between streams of the same size. Multiple regression of species richness, diversity, abundance and biomass data on principal components revealed significant but low correlations with measured habitat variables. Riffle habitats showed lower species richness and diversity but high abundance. The fish assemblage in riffles was dominated by balitorid species, specialized for fast-water conditions. Pool assemblages had the highest species diversity and were dominated by cyprinid species of a number of morphological and ecological guilds. Run assemblages were intermediate in assemblage characteristics between riffle and pool assemblages. Between-stream variation in assemblage composition was less than within-stream variation. Of 38 species collected, seven could be designated as riffle specialists, 18 as pool specialists and 13 as ubiquitous, although most of the latter showed size-specific habitat use with larger size classes found in slower, deeper water.     

Temporal patterns of grazer-periphyton interactions in laboratory streams

SUMMARY. 1. The snail Juga silicuta (500 m^?2) and the caddisfly Dicosmoecus gilvipes (50 m^?2) were introduced into separate laboratory streams on days 1, 9, 16 and 28 of algal development. The mayfly Baetis spp. (500 m^?2) was introduced on days 1 and 16, and two streams did not receive grazers. We assessed the interaction between succession in the pcriphyton, herbivore type and time of encounter in a 40-day experiment. 2. In ungrazed streams, the chlorophyte Scenedesmus obliquus was the most abundant early colonizer. The relative abundance of diatoms increased after day 9, and at day 40 the algal assemblage consisted of a thick mat of diatoms and S. obliquus with an overstorey of filaments of the chlorophyte Stigeoclonium tenue. In general, introductions of grazers at any stage altered this pattern by removing biomass, accelerating the replacement of S. obliquus by diatoms, and suppressing the growth of filaments. Grazing also reduced the relative abundance of the larger diatom Nitzschia oregona but increased the relative abundance of the smaller adnate diatoms Nitzschia frustulum var. perpusilla and Navicula minima. 3. Dicosmoecus decreased algal biomass and altered successional trajectories to a greater degree than either Juga or Baetis. Dicosmoecus rapidly grazed the entire substrate, whereas Juga and Baetis only cleared patches in the assemblages. Little alteration in algal development was observed in the Baetis streams after day 16, probably because (he periphyton assemblages attained a size and structure that prevented effective grazing by Baetis. 4. The patchy grazing patterns of Juga and Baetis resulted in more diverse algal assemblages than either the Dicosmoecus grazed or ungrazed streams. In natural streams, the temporal and spatial pattern of grazing relative to the developmental stage of the periphyton may contribute to maintaining a mosaic of algal patches in different serai stages.     

Fishes of Indiana streams: current and historic assemblage structure

We examined Indiana fish assemblages using taxonomy and ecological categories to assess temporal shifts in community structure and recent environmental relationships. Historic (1945) and recent (1996–2007) presence/absence data were compiled by subbasin and analyzed with Nonmetric Multidimensional Scaling (NMS) ordination and by species richness. Canonical Correspondence Analysis (CCA) was used to test taxonomic identity and ecological category abundance data for explanation with recent (1996–2007) environmental variables. We found a decrease in assemblage heterogeneity for recent assemblages and an increase in the number of tolerant species per subbasin. Recent Indiana streams are dominated by tolerant fishes with generalist life history strategies and low functional variation. The use of ecological categories resulted in weaker relationships with environmental variables than analyses with taxonomic identities. Analyses using taxonomy resulted in strong assemblage explanation from stream size and flow variation, while analyses using ecological categories resulted in strong assemblage explanation from habitat variation in silt substrates and flow. Analyses of recent assemblage structure using ecological categories resulted in decreased assemblage variation among subbasins than in analyses using taxonomic identities. We found that fish assemblages of Indiana streams are structured primarily by habitat complexity and have been altered during the past 50 years through multiple disturbances including fragmentation, siltation, and species introductions.     

EFFECTS OF SUBSTRATE RELIEF ON THE DISTRIBUTION OF PERIPHYTON IN LABORATORY STREAMS. II. INTERACTIONS WITH IRRADIANCE1

Laboratory streams were used in a 42-day experiment designed to investigate how the spatial and temporal distribution of lotic periphyton created by current flow over cobble-size substrates is a affected by irradiance. The streams contained 22.5 × 22.5 × 4 cm substrate blocks and were exposed to either 385, 90 or 20 μE·m^?2·s^?1. We monitored periphyton succession in fast current regimes on top of blocks and in slower current regimes on surfaces recessed between blocks. The absolute differences in AFDW algal biomass between top and recessed substrates were significantly affected by irradiance and time. At the end of the experiment, biomass in streams exposed to 385 μE·m^?2·s^?1. was approximately 2 and 8 times greater than in streams exposed to 90 and 20 μE·m^?2·s^?1, respectively. Differences in biomass were greater between irradiance levels than between top and recessed substrates within an irradiance level. Irradiance also had a greater effect than current regime on the taxonomic composition of assemblages. Oscillatoria agardhii Gomont and Navicula minima Grun. characterized assemblages at 20 μE·m^?2·s^?1, whereas Fragilaria vaucheriae (Kütz.), Nitzschia oregona Sov., Navicula arvensis Hust. and Stigeoclonium tenue (Ag.) Kütz. were more abundant at the two higher irradiances. Detrended correspondence analysis indicated that the rate of succession was relatively high for assemblages at high irradiance and in the slow current regimes between blocks. The results suggested that in natural streams, periphyton patches produced by large differences in irradiance should have a greater effect on periphyton heterogeneity than substrate-induced patches. Moreover, the heterogeneity of algal patches produced by hydrologic differences over a substrate is constrained by irradiance level.     

The effects of predation by juvenile fish on the meiobenthic community structure in a natural pond

相似文献   

Top-down control of a meiobenthic community by two juvenile freshwater fish species

Top-down control of prey assemblages by fish predation has been clearly demonstrated for zooplankton and macroinvertebrates. However, in the benthic communities of freshwater ecosystems, the impact of fish predation on meiofaunal assemblages is nearly unknown. In this study, the predation effects of juvenile carp (Cyprinus carpio) and gudgeon (Gobio gobio) on meiofaunal abundance, biomass, community structure, and the diversity of nematodes were examined using microcosms that were sampled repeatedly over 64 days. Significant differences in abundance and biomass were found between the two fish treatments (carp and gudgeon) and their respective controls for nematodes, oligochaetes, and crustaceans (copepods, harpacticoids, ostracods, and cladocerans), but not for rotifers. These changes were consistent with top-down control of the freshwater meiofaunal assemblages in the microcosms over time. By contrast, small-bodied meiofauna was more abundant, suggesting indirect facilitation. Neither the species richness nor the diversity of the nematode community was affected by fish predation. The results indicate that predation by juvenile freshwater fish depresses the overall abundance and biomass of meiofaunal assemblages, except for rotifers, and alters the size structure of the meiofaunal community. Therefore, the meiofaunal assemblages of freshwater ecosystems may be influenced by bottom-feeding juvenile fish, e.g., carp and gudgeon, through top-down control of meiofaunal populations.     

The productivity and carbon budget of a natural population of Daphnia lumholtzi Sars

D. lumholtzi in Lake Samsonvale, Queensland, Australia, is a small species (max. size approx. 7 µgC) that occurs in low abundance (max. abundance 6400 m^–3), with an average daily biomass of 3.32 mgC m^–3. Its annual rates of carbon assimilation, production and respiration, are 166, 110, and 56 mgC m^–3 y^–1 respectively. Annual biomass turnover (annual production/average daily biomass) is 33 and production efficiency is 50–66%. The population may consume 1.65–2.20 mgC m^–3 daily, equivalent to about 1% of the average daily standing crop of phytoplankton. Clutch size is small, 2 eggs, but represents 30–80% of a female's weight. A female may only produce 8–10 offspring in a full lifespan, nevertheless egg production may account for 56% of total production. The population shows autumn and spring peaks in abundance, and is believed to oversummer (4 months) as ephippia.     

EFFECTS OF IRRADIANCE AND GRAZING ON LOTIC ALGAL ASSEMBLAGES1

A laboratory experiment was conducted for 75 days to examine how irradiance levels and grazing influence algal biomass and community structure. Twelve laboratory streams were used for experimental analyses, with four channels exposed to one of three irradiance levels (15, 100, or 400 μE·m^?2·s^?1). Three of the four stream at each light level were stocked with the snail Juga silicula (250·m^?2), leaving one stream at each light level without snails. Grazed stream exposed to low light levels developed low amounts of algal biomass (<2 g AFDW·m^?2) and were dominated by adnately attached diatoms. Mean algal biomass increased over time in the grazed streams exposed to intermediate light; by day 75, these streams were characterized by moderate algal biomasses (30-40 g AFDW·m^?2) and filamentous chlorophytes. Algal assemblages in high light, grazed channels had high levels of biomass at day 43 (70 g AFDW·m^?2) that declined to 30 g AFDW·m^?2at day 75 and were dominated by chlorophytes. Among ungrazed streams, algal biomass at day 75 was relatively low in the low light streams (<7g AFDW·m^?2) and was dominated by adnately attached diatoms. Ungrazed streams exposed to intermediate and high light levels had moderate biomasses (23 and 19 g AFDW·m^?2, respectively) and were dominated by chlorophytes and large diatoms. Grazing appeared both to delay and alter successional trajectories of algal assemblages, with alterations most noticeable during early seral stages at intermediate and high light levels. Grazing had the least effect on successional trajectories at low light.     

Abundance and biomass of heterotrophic microorganisms in Lake Tanganyika

1. This study focused on heterotrophic microorganisms in the two main basins (north and south) of Lake Tanganyika during dry and wet seasons in 2002. Bacteria (81% cocci) were abundant (2.28–5.30 × 10⁶ cells mL^?1). During the dry season, in the south basin, bacterial biomass reached a maximum of 2.27 g C m^?2 and phytoplankton biomass was 3.75 g C m^?2 (integrated over a water column of 100 m). 2. Protozoan abundance was constituted of 99% of heterotrophic nanoflagellates (HNF). Communities of flagellates and bacteria consisted of very small but numerous cells. Flagellates were often the main planktonic compartment, with a biomass of 3.42–4.43 g C m^?2. Flagellate biomass was in the same range and often higher than the total autotrophic biomass (1.60–4.72 g C m^?2). 3. Total autotrophic carbon was partly sustained by the endosymbiotic zoochlorellae Strombidium. These ciliates were present only in the euphotic zone and usually contributed most of the biomass of ciliates. 4. Total heterotrophic ciliate biomass ranged between 0.35 and 0.44 g C m^?2. In 2002, heterotrophic microorganisms consisting of bacteria, flagellates and ciliates represented a large fraction of plankton. These results support the hypothesis that the microbial food web contributes to the high productivity of Lake Tanganyika. 5. As the sole source of carbon in the pelagic zone of this large lake is phytoplankton production, planktonic heterotrophs ultimately depend on autochthonous organic carbon, most probably dissolved organic carbon (DOC) from algal excretion.     

Seasonal changes in the distribution and abundance of benthic invertebrates in six headwater streams in central Florida

Seasonal variations in invertebrate assemblages at two sites (upstream and downstream) on six central Florida headwater streams were compared by sampling at quarterly intervals with core and dip net samplers. Two of the streams were reclaimed following phosphate mining (～6 yr prior to this study), two received runoff from mined lands, and two were disturbed by agriculture and/or residential developments. Physical and chemical characteristics of the reclaimed streams differed markedly from those of the non-reclaimed streams; principal differences between the streams were in current velocity, percent organic matter (POM), Mn, conductivity and alkalinity. Annual mean densities of meiofauna and smaller macrofauna for the 12 stream sites ranged from 20?896 to 175?212 m^?2 and the mean for all sites was 56?492 m^?2. The reclaimed streams and one of the streams influenced by agriculture had annual means of less than 40?000 m^?2, 3- to 5-fold lower than the other streams. Fall and winter core densities were ～2.4-fold greater than those for spring or summer when drought and low dissolved oxygen prevailed. Meiofauna comprised 68–91% of the core sample invertebrates in reclaimed streams but only 43–62% in the non-reclaimed streams; principal functional groups were: gathering collectors – 61.5%, predators – 19.3% and filtering collectors – 15%. The taxonomic composition of the reclaimed streams was predominated by crustaceans (60–71%) while chironomids and annelids were more abundant (71–92%) in the non-reclaimed streams. Dip net sampling added 21 larger macrofauna species (Odonata, Hemiptera and Coleoptera) to our list of taxa, producing a total of 209 species. Species richness and diversity (H′ and N ₂) indices were lower in the reclaimed streams, but evenness was more variable. The Czekanowski–Dice–Sørensen similarity index showed that the reclaimed stream sites were quite similar to each other, but differed markedly from the other stream types; there was large variation both within and between seasons. For central Florida headwater streams, drought appears to have a larger influence on invertebrates than the type of land use, however this relationship should be confirmed using streams of similar hydrology.     

Macroinvertebrate stream communities along regional and physico-chemical gradients in Western Greenland

1. Macroinvertebrates were collected and physico‐chemical variables measured at 16 stream sites in Western Greenland during July 1999. Eight sites were located on Disko Island in an arctic oceanic climate and eight sites in the Kangerlussuaq area close to the icecap where the climate is arctic continental. The streams had different water sources (glacial, groundwater, snowmelt and lake water). 2. The streams showed pronounced differences in water temperature (2.2–17.3 °C), concentrations of suspended solids (0–2400 mg L^?1), and conductivity (10–109 μS cm^?1). Principal component analysis (PCA) analysis of the physico‐chemical variables separated the Disko Island sites into a distinct group, whereas the sites in the Kangerlussuaq area were more dispersed. 3. A total of 56 macroinvertebrate species were found, including 31 species of Chironomidae, the most abundant of which was Orthocladius thienemanni. Diamesa sp. was only the sixth most abundant chironomid taxon. Species composition varied between sites, and abundance varied from about 20 individuals m^?2 in a glacier fed stream to more than 16 000 m^?2 in a lake outlet. 4. The macroinvertebrate communities of the 16 streams were separated into five TWINSPAN groups reflecting water source, irrespective of region. Lake outlets and ground‐water‐fed streams had the highest species richness and abundance, temperature and bed stability, while glacier‐fed streams were characterized by low species richness, abundance, temperature, bed stability and high concentrations of suspended solids. Macroinvertebrate species richness was positively correlated with water temperature and negatively with bed stability. Conductivity was positively correlated with invertebrate abundance. 5. The results of this study suggest that the source of stream water can be used to predict invertebrate community composition in Greenlandic streams and thus the effects of changes in water balance and flow regime, and to identify sites of special conservation interest.     

Salinity and fish effects on Salton Sea microecosystems: benthos

The Salton Sea, the largest lake in California, has a surface elevation 69 m below sea level which is maintained predominantly by the balance of agricultural runoff and evaporation. The lack of outflowing streams is resulting in a gradual buildup of salts in the lake, increasing the salinity. A 15 month microcosm experiment was conducted to determine the effects of salinity and tilapia ( Oreochromis mossambicus) on an assemblage of benthic and planktonic Salton Sea algae and invertebrates. This article reports the responses of the benthic invertebrates. Microcosms (312 l fiberglass tanks) were set up without tilapia at 30, 39, 48, 57, and 65 g · l-1. Additional microcosms were set up with tilapia at 39 and 57 g · l-1. In the absence of fish Gammarus mucronatus dominated the benthos at the lower salinities, and Trichocorixa reticulata and the larvae of Ephydra riparia were most abundant above 48 g · l-1. The most abundant meiofaunal species included the harpacticoid copepod. Cletocamptus deitersi, three nematodes, the rotifer Brachionus plicatilis, ciliates, including Condylosoma sp. and Fabrea salina, two foraminiferans including Quinqueloculina sp., and a large flagellate. Most meiofaunal species responding to salinity were most abundant at 65 g · l-1, especialy after 6 months when Gammarus dominated the lower salinities. The tilapia reduced the abundance of macrofaunal species, especially at 39 g · l-1, and generally increased the abundance of meiofaunal species and ciliates. The microcosm benthic macro- and meiofaunal communities were most likely structured by Gammarus, salinity and tilapia. Gammarus reduced the other species by predation and changing the detritus from an algal base to a fecal pellet base. Gammarus was itself reduced by tilapia and by reduced reproductive success above 39 g · l-1. More species were therefore able to compete at higher salinities and in the presence of tilapia. Tilapia also affected the benthos by depositing loosely packaged fecal material which may support more meiofaunal species than either the robust Gammarus fecal pellets that were abundant at 39 g · l-1 or the algae-fecal pellet mix at 57 g · l-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spatial and seasonal variations in benthic algal assemblages in streams in monsoonal Hong Kong

Samples from stone surfaces were collected in pools within four unpolluted hillstreams (two shaded and two unshaded) in monsoonal Hong Kong (lat. 23°N) to elucidate the extent of spatial (within and among streams) and temporal (seasonal) variations in algal biomass and assemblage composition. Sampling continued for over 12 months, incorporating the dry season when streams were at baseflow, and the wet season when spates were frequent. We anticipated that algal biomass would be lower in shaded streams and during the wet season, with associated seasonal differences in assemblage composition or relative abundance of different growth forms (e.g. erect versus prostrate). Benthic chlorophyll a (a proxy for algal biomass) varied among streams from an annual mean of 11.0–22.3 mg m⁻². Dry-season standing stocks were 18% higher than during the wet season when spate-induced disturbance reduced algal standing stocks. Algal biomass varied significantly at the stream scale, but not at the pool scale, and was lower in unshaded streams, where standing stocks may have been limited by high densities of algivorous balitorid loaches (mainly Pseudogastromyzon myersi). An overriding effect of grazers on algal biomass could also have reduced variations resulting from spate-induced disturbance. Significant differences in assemblage composition among streams, which were dominated by diatoms and cyanobacteria (totally 82 taxa) were not systematically related to shading conditions. Seasonal variations in algal assemblages were statistically significant but rather minor, and did not involve major shifts in composition or growth form caused by spate-induced disturbance. The abundance of filamentous cyanobacteria in all the streams may have been due to ‘gardening’ by balitorid loaches that removed erect or stalked diatoms and favoured cyanobacteria that persist through basal regeneration of filaments. This explanation requires validation through manipulative experiments. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Luis Mauricio Bini     

Macrophyte beds contribute disproportionately to benthic invertebrate abundance and biomass in a sand plains stream

Macrophyte beds have been shown to influence organic matter retention and nutrient processing in streams. Less is known about the extent to which plant beds contribute to abundance, biomass, and diversity of macroinvertebrate assemblages in low-order streams. We measured aquatic invertebrate abundance, biomass, and diversity associated with plant beds and sand/gravel patches in a low-gradient second-order stream in the Central Sand Plains of Wisconsin, USA from March to October. Invertebrate abundance and biomass were higher on average in plant beds (2,552 m⁻² and 1,575 mg m⁻²) than in sand/gravel patches (893 m⁻² and 486 mg m⁻²). Although sand/gravel habitat was over three times more abundant than plant beds in the study reach, plant beds and sand/gravel patches contributed similarly to invertebrate abundance and biomass at the whole-reach scale. The abundance and biomass of invertebrates associated with plant beds decreased from spring to autumn. Non-insect invertebrates in the plant beds increased in relative abundance as the year progressed. Shannon–Weiner diversity and taxa richness of invertebrates were higher in the plant beds than in the sand/gravel habitat. Our results suggest that plant beds can represent hot spots for invertebrate abundance and production in low-gradient streams, and have implications for stream management and restoration in these types of ecosystems. Handling editor: S. I. Dodson     

Cascading effects of predatory fish on the composition of benthic algae in high‐altitude streams

Cascading effects of predators can affect ecosystem properties by changing plant biomass, distribution and assemblage composition. Using data from field surveys and whole‐stream experiments we tested the hypothesis that predatory trout change assemblage composition of benthic algae in high‐elevation streams mediated by grazer behavior. Field surveys revealed that the taxonomic composition of algal assemblages differed significantly between streams that contained trout and those that were fishless; but comparisons of palatable versus unpalatable algal taxa between fish and fishless streams were equivocal because of high natural variability. Therefore, we tested for a behavioral (non‐consumptive) trophic cascade experimentally by adding brook trout chemical cues to six naturally fishless streams for 25 days and compared responses of grazers and algae to six reference streams without fish cues added. Algal response variables included rates of change in the abundance of three physiognomic categories, from most palatable (attached erect and prostrate diatoms) to least palatable (non‐diatoms), as determined from food selectivity analyses of the most common grazers (mayflies and caddisflies). Fish cues did not affect the mean densities or changes in densities of total grazers or any individual grazer species. However, in streams where fish cues were added, rates of accrual of attached erect diatoms, which was the preferred algal type for the grazer most vulnerable to trout predation (Baetis), were higher and their densities increased significantly faster with increasing densities of this grazer species than in reference streams. Results of his experiment support the hypothesis that predator induced suppression of grazer foraging behavior, rather than cascading effects of top predators on grazer density, may contribute to variation in the composition of algal assemblages among streams by allowing proliferation of most palatable algal species.     

EFFECTS OF ISLAND GROUPS,DEPTH, AND EXPOSURE TO OCEAN WAVES ON SUBTIDAL MACROALGAL ASSEMBLAGES IN THE RECHERCHE ARCHIPELAGO,WESTERN AUSTRALIA1

Temperate Australia has a speciose highly endemic algal flora. This study explored the influence of geographical isolation between islands, depth and exposure to ocean swells on the diversity of macroalgae in the Recherche Archipelago (Western Australia). Macroalgae were harvested (0.25‐m² quadrats) from sites at two exposures (sheltered and exposed to wave energy), three depths (<10, 10–20, and 21–28 m), and two island groups (three islands within Esperance Bay and three islands outside the bay). A total of 220 species were collected. Species richness and biomass were significantly different at the smallest spatial scale (0.25 m²), and density of overstory species decreased with depth. Results from analysis of similarity tests suggested that macroalgal assemblages differed with depth, exposure and to a lesser extent with island group. Assemblage differences were often associated with particular overstory or understory taxa and not the entire assemblage composition. Average species richness·0.25 m^?2 ranged from 13 to 29 species, typically with a few species contributing more than 50% of average biomass. Species richness was maintained by species turnover at the 0.25‐m² spatial scale. Our results suggest that richness in temperate Australia is maintained by turnover of broadly distributed species. More dominant species in assemblages were associated with differences in depth and exposure to ocean swells. Our findings support the hypothesis of a geographical transition of dominant species from kelp‐dominated in the west to a fucalean‐dominated assemblage in the Recherche Archipelago and east of the Great Australian Bight.