Structure Of Algal Assemblages Research Articles

Community responses to UV radiation. I. Enhanced UVB effects on biomass and community structure of filamentous algal assemblages growing in a coral reef mesocosm

Four treatments (PAR; PAR+UVA; PAR + UVA+UVB enhanced by 20% from ambient levels; ambient levels of UVB) were performed using a combination of metal halide lights, UV lamps and cutoff filters over developing assemblages of filamentous algae in a coral reef mesocosm. Exposure to enhanced UVB initially reduced the standing crop by 67% as compared to the productivity of the assemblages grown under PAR and PAR+UVA. Treatment reversal from PAR to enhanced UVB restricted temporarily the growth of the brown alga Ectocarpus rhodochondroides. While the spores of this species are inhibited by UVB, the sporophytes seem to be capable of adapting to UVB exposure. The effects of ambient UVB levels on biomass production and community composition were less pronounced, while exposure to UVA did not affect productivity or community composition. All effects due to UVB exposure gradually diminished as succession progressed. Community composition and biomass production were fully restored 1 to 2 weeks after the cessation of exposure to enhanced UVB.

Temporal variation in plankton assemblages and physicochemistry of Devils Lake, North Dakota

Seasonal and annual variation in biomass and structure of algal assemblages of hyposaline Devils Lake were examined in relation to turbidity, ambient concentrations of major ions, trace elements and nutrients, and the standing crop of herbivores. Lake level declined during the early years of study, but rose markedly in subsequent years as historically large volumes of water flowed into this hydrologically-closed basin. Winter algal assemblages were dominated (in biomass) most years by small, non-motile chlorophytes ( Choricystis minor, Kirchneriella lunaris or Dunaliella sp.), or Euglena sp. in the most saline sub-basin. Spring assemblages were dominated by diatoms (Stephanodiscus cf. minutulus, Surirella peisonis, Cyclotella meneghiniana and Entomoneis paludosa were especially prominent) or chlorophytes ( C. minor) until the lake level rose. C. minor abundances then declined in spring assemblages and diatoms ( Stephanodiscus cf. agassizensis and S. niagarae; E. paludosa in the more saline sub-basins) dominated. The potential for nitrogen-deficient conditions for phytoplankton growth was evidenced most summers and early autumns by consistently high concentrations of reactive-P relative to inorganic-N and blooms of the N-fixing cyanophyte Aphanizomenon flos-aquae; Microcystis aeruginosa typically was a co-dominant (>30% of biomass) in these assemblages. Pulses of diatoms ( S. cf. agassizensis and C. meneghiniana) occurred in summers following unusually prolonged periods of calm weather or large water inflows. Physical (irradiance, turbulence) and chemical (major nutrients) variables were the primary factors associated with phytoplankton growth. Transparency and major nutrient concentrations accounted for more of the annual variation in phytoplankton structure than did salinity. Seasonal abundance patterns of the dominant zooplankton (the copepod Diaptomus sicilis; the cladocerans Ceriodaphnia quadrangula, Chydorus sphaericus, Daphnia pulex and Diaphanosoma birgei; and the rotifers Brachionus spp., Filinia longiseta, Keratella cochlearis and K. quadrata) also indicated variation in algal populations related to grazing.

Inducible defence against a ciliate grazerPseudomicrothorax dubius, in two strains ofPhormidium(cyanobacteria)

Experiments were done with two strain of filamentous, mat-forming Phormidium and their ciliate grazer Pseudomicrothorax dubius, to explain why the ciliates remain hungry in an apparent surplus of food, except for the first 24 hours after feeding. Under grazing pressure, both strains of cyanobacteria showed statistically significant increases in the number of filaments terminating in an empty sheath, compared to the control. Direct observations revealed that the mechanism behind this effect was active withdrawal of the trichomes inside the sheaths when disturbed by grazers. As P. dubius is unable to ingest trichomes with such endings, we conclude that cyanobacteria are not limited to chemical means of defence against grazers but can also defend themselves by means of movement and changes in filament morphology. This is apparently the first report on behavioural defence observed in cyanobacteria.

Resource Thresholds and Stream Ecosystem Sustainability

Many factors contribute to the complex patterns of ecological conditions in streams. Nutrient and light availability are 2 factors that directly affect benthic algal assemblages and thereby indirectly affect stream faunas. A null model of the major changes and determinants of benthic algal assemblage structure and function along resource (nutrient and light) gradients is proposed. This model predicts that low-resource stress, herbivory, competition, and colonization of habitats by new algal growth forms are the dominant determinants of benthic algal ecology at successively higher resource levels. It also predicts major changes in algal assemblages in resource conditions at which regulation of communities changes from one dominant determinant to another. In many streams, diatoms dominate low-density biofilms in low resource conditions where resource stress and herbivory regulate periphyton. Above a herbivory threshold, a resource condition in which algal accumulation rates equal grazing rates, diatoms should accrue to densities that are regulated by competition for light and nutrients. Another resource threshold can occur where increasing nutrient or light regimes make the habitat available for colonization by different algae, such as Cladophora or other filamentous greens, that have growth forms very different from those of diatom-dominated assemblages. A concept of ecological sustainability is also proposed which quantifies the changes in environmental conditions that an ecosystem can withstand without "significantly" changing ecological integrity. Linkage between the resource threshold hypothesis and ecological sustainability helps to define environmental criteria at points along resource gradients where significant changes in ecological processes are expected.

Scale-Dependent Determinants and Consequences of Benthic Algal Heterogeneity

The objectives of this paper were to introduce frameworks for predicting the determinants and consequences of the great heterogeneity in benthic algal assemblages. A hierarchical framework of environmental factors affecting benthic algal spatial heterogeneity is presented with the ultimate determinants (climate, geology, land use, and biogeography) that are important constraints on benthic algal assemblage structure at large spatial scales. These ultimate determinants constrain expression of intermediate and proximate determinants of benthic algal function and structure. A similar framework for temporal heterogeneity distinguishes how assemblages respond to short-term and long-term environmental changes, referred to as disturbances and stresses, respectively. Assemblages recover from punctuated (short-term) environmental change (disturbance) by immigration and reproduction of both persistent and recolonizing species. Assemblages adapt to permanent (longterm) environmental change (stress) by changing speci...

Herbivory under Different Flow Regimes: A Field Experiment and Test of a Model with a Benthic Stream Insect

An experiment was conducted in an open-canopy mountain stream to examine the effects of different densities of the grazing caddisfly Agapetus boulderensis (Trichoptera: Glossosomatidae) on algal biomass and assemblage structure at two current velocities. Unglazed ceramic tiles were colonized by algae in grazer-free troughs for 30 d at slow (∼ 20 cm s −1 ) and fast (∼ 40 cm s −1 ) velocities before transfer to open troughs of similar velocity, where they were exposed to grazer densities ranging from ca 0.7 to 2.7 times natural streambed densities. At low velocity, algal biomass (chl-a and particulate C) and absolute densities of bacillariophytes, chlorophytes and cyanophytes declined as grazer density increased. Grazers also significantly altered algal assemblage structure by reducing the relative abundance of cyanophytes (mostly Anabaena cf. affinis) and chlorophytes (Ulothrix zonata) and increasing the relative abundance of some bacillariophytes (Cocconeis placentula, Fragilaria pinnata, F. vaucheriae, Melosira italica). The algal assemblage was effectively converted from a «late-successional» to an «early-successional» stage by grazers. By contrast, at high velocity, initial algal density was relatively low and grazers were less effective in inducing changes. As grazer density increased, total algal cell abundance (mostly bacillariophytes) declined, although the relative abundances of individual diatom species remained unmodified under increasing grazer densities. Grazers essentially maintained the «early-successional» nature of the algal assemblage under high velocity conditions. Further, for both current velocities, we tested a model that predicts algal biomass as a function of grazer density and time. Independent estimates of current-dependent grazing rate and algal growth rate were used in the model to generate predictions of particulate C along a gradient of grazer density at two current velocities. Algal biomass predicted by the model compared favorably with an empirical fit of the data under low and high velocities. These results suggest that the role of grazers in structuring algal assemblages may vary spatially with local current velocity in heterogeneous stream systems

Structure of subtidal algal assemblages on soft-bottom sediments: fauna/flora interactions and role of disturbances in the Bay of Brest, France

Algal and fauna1 assemblages living on soft bottom sediments were studied in the Bay of Brest, France, using quantitative descriptions of the distribution, diversity and biomass of epibenthic species. Similar classification and ordination analyses were used on floral and animal data. Dense assemblages of macrophytes, mainly Rhodophyceae, were found to grow on sandy mud sediments. Sediments were essentially covered by maerl beds of the calcareous red alga Lithothamnion corallioides and various shell substrates. Five maln algal assemblages extended along a west-east decreasing gradient of hydrodynamic activity (i.e. tidal currents, waves and turbidity). The high frequency of storms during the year is the main feature which disturbs both flora and epifauna. Interactions between flora and fauna were either direct, e.g. grazing and spatial competition, or indirect, most of the substrata occupied by macrophytes being shells of dead bivalves and gastropods. Moreover attachment of algae was largely dependent on population dynamics of shell species living in the area. As a consequence, in the most disturbed area, the macrophytes found were opportunistic species (Polysiphonia fibrillosa and Polysiphonia urceolata). In areas where the assemblage was unstructured, a fact which prevented the development of the herbivorous species, the animal assemblage was then dominated by suspension feeders (FicuLina ficus and Phallusia marnmillata) which competed for space with the macrophytes. In the least disturbed area, the herbivorous species dominated the fauna assemblage while flora assemblage had a high level of organization. However, in those places eutrophication and high turbidity prevented the establishment of many algal species, causing the ubiquitous ones to domInate (Ulva sp., Ceramium rubrum). Between these extreme areas, the living Lithotharnnion beds were more developed and were defined as a characteristic assemblage of diversified Rhodophyceae species such as Chyloclad~a verticillata, Halarachnion ligulaturn, Crytopleura ramosa. Finally, the role of wave disturbance as the main factor controlling algal assemblages of shallow soft-bottom sedlments is discussed.

Lotic Protistan Herbivore Selectivity and Its Potential Impact on Benthic Algal Assemblages

Despite the ubiquitous presence of microscopic herbivores such as protozoa in aquatic ecosystems, research on benthic herbivory has largely focused on the role of macroscopic grazers. The present study shows the potential for protozoan herbivores to affect the structure of benthic algal assemblages through selective grazing. The abundance and feeding preferences of three common algivorous protozoans, the ciliates Chilodonella sp. and Trithigmostoma cucullulus and the sarcodine Pelomyxa sp., were determined in a productive, third-order stream and in stream-side channels which contained different densities of two macrograzers and algal assemblages with different physiognomies and species composition. Although the species composition of the ingested algal contents of individual micrograzer cells varied, all three micrograzer populations exhibited selectivity in their feeding on dominant diatom species. Selectivity varied among microherbivore species and with the abundance of indigenous macrograzers. Although...

Algal Colonization under Four Experimentally-Controlled Current Regimes in High Mountain Stream

Algal colonization and assemblage development were examined on unglazed ceramic tiles in flow-through troughs over a 42-d period (summer 1987) in the regulated upper Colorado River (∼2400 m a.s.l.). The unusually constant discharge of the stream, coupled with the trough design, allowed the establishment and maintenance of four controlled current regimes that differed with respect to range of free-stream velocity and associated near-substratum hydraulic conditions: 1) depositional (free-stream current velocity <1 cm/s); 2) slow and hydraulically-smooth (range 14-20 cm/s); 3) fast and hydraulically-smooth (range 36-47 cm/s); and, 4) fast and hydraulically-rough or turbulently mixed (range 15-46 cm/s). Biomass was on average 30-40× higher in the two slow velocity treatments than in the two fast velocity treatments. Chlorophytes, bacillariophytes, and cyanophytes exhibited distinctly different successional trajectories and end-points in all treatments. Ulothrix zonata, established by day 4, was the dominant alga at all velocities for the first two weeks, after which it was replaced by a more diverse assemblage of chlorophytes, except in the Fast-rough regime. Diatoms were common in all treatments, but were only numerically dominant during the latter part of the study. Cyanophytes dominated only in the two lowest current regimes and only at the end of the study. Scanning electron microscopy revealed that physiognomy of algal assemblages also varied across current regimes. The two low velocity treatments were characterized by dense, upright filaments within one week, while three-dimensional assemblage structure in the two high velocity treatments did not develop over the 42-d period. Differences in species composition, successional trajectories and physiognomy across treatments demonstrated that current regime (free-stream velocity and pattern of flow) was an important determinant of algal assemblage structure in the absence of hydrologic and grazing disturbances.

Structure and Composition of Ice Algal Assemblages from the Gulf of St. Lawrence, Magdalen Islands Area

Two types of ice algal assemblages were found in the Gulf of St. Lawrence: assemblages composed predominantly of pennate diatoms (abundance &gt; 98.0%) and assemblages with a high abundance of centric diatoms (abundance &gt; 46.2%). The first type is similar to Arctic landfast ice algal assemblages with the pennate diatoms Nitzschia cylindrus, N. polaris, and Navicula kariana as dominant species. The second type is similar to drifting ice algal assemblages previously described from the Gulf of St. Lawrence, with a percentage of centric diatom species &gt; 46.2% due to the dominance of the planktonic diatom Thalassiosira nordenskioeldii. Species richness and the Shannon-Weaver index of diversity were low; density ranged from 104–106 × cells∙L−1 and was negatively correlated with percent centric diatoms. Results on the structure and the composition of the ice algal assemblages are related to ice type, i.e. landfast and drifting pack ice, and compared with ice algal assemblages from higher latitudes.

Ecology of marine algae on rocky shores and subtidal reefs in temperate Australia

Experiments in intertidal and subtidal rocky marine habitats in temperate Australia have identified the effects of various biological and physical factors on algal assemblages. In intertidal habitats, these involve micro- and macro-algae and grazing by gastropods. In subtidal habitats, interactions among micro- and macro-algae, echinoids, gastropods, micro-invertebrates and sessile invertebrates have been studied. Experimental studies on physical disturbances of algal assemblages have focussed on the effects of desiccation and storms. Most studies have not considered more than one spatial or temporal scale. Few have been concerned with seasonal influences and fewer have been concerned with variation from year to year. Most of the work lacks applicability to biogeographic comparisons. More experimental work across a variety of spatial and temporal scales is required to determine significant biological and physical processes affecting structure of algal assemblages across broad areas of temperate Australia.

Organization of macroalgal assemblages in the Northeast Pacific: the assumption of homogeneity and the illusion of generality

Evidence for the geographic generality of the causes of intertidal zonation and the indirect effects of a ‘keystone’ predator, the sea otter, on subtidal kelp assemblages was examined. Most research on intertidal algal assemblages has been done at a few protected sites where zonation is distinct. Surveys of wave-exposed intertidal sites in central and northern California show that assemblage structure is highly variable. This indicates that our present understanding of assemblage organization, including the effects of mussel-algal interactions, may not be widely applicable. Surveys of kelp forest habitat along the entire coast of California suggest that deforestation by sea urchins is uncommon in the absence of sea otters. These examples indicate that the generality of commonly accepted causes of algal assemblage structure in the Northeast Pacific may be an illusion based on assumptions of environmental homogeneity.

Productive Capacity of Periphyton as a Determinant of Plant-Herbivore Interactions in Streams

To investigate the influence of plant productivity on plant—herbivore interactions in stream ecosystems, we varied the productive capacity of algal assemblages by exposing periphyton to three levels of irradiance and two levels of grazing. We studied interactions between algal assemblages (grown from algae obtained from four Oregon streams) and herbivorous snails (Juga silicula) in 15 laboratory streams containing either 250 snails/m2 or no snails. Biomass, production, export, and taxonomic structure of the algal community were measured at intervals throughout the 75—d study. Ingestion rate and assimilation efficiency of snails also were measured on six different dates using dual—isotope labeling, and snail growth was measured at the end of the experiment. Rates of primary production, algal biomass accumulation, and dominance by chlorophytes generally increased with higher irradiance, although these patterns were modified by herbivores. Ungrazed periphyton at low irradiance (photon flux density: 20μmol°m—2°s—1) accumulated little biomass, which was further reduced by grazing snails. At intemediate (100 μmol°m—2°s—1) and high (400μmol°m—2°s—1) irradiance, snails delayed the accumulation of algal biomass but did not affect the final biomass attained. After 43 d, net primary production (NPP) at high irradiance was unaffected by grazing, whereas grazing increased NPP at both low and intermediate irradiance. Algal export increased with both irradiance and the presence of grazers and constituted a significant loss of plant biomass from the streams. Grazing by Juga delayed algal succession and altered algal taxonomic structure and assemblage physiognomy by reducing the relative abundance of erect and non—attached algae, while increasing the abundance of adnate diatoms. Snails grew slowly at low irradiance, due to scant food resources, but had high growth rates at intermediate and high irradiance, probably because food was not limiting. Assimilation efficiencies for snails generally varied from 40 to 70% and were highest at low irradiance. At low irradiance, 90% of benthic production was harvested by grazers, whereas only 10% accumulated as attached biomass or was exported. At higher irradiances, <15% of primary production was harvested by grazers, and >85% persisted as attached algae or was exported. In these stream ecosystems, the biomass and production of grazers were influenced by abiotic constriants placed on algal productive capacity (i.e., the ability of a plant assemblage to generate biomass). The structure and metabolism of algal assemblages were affected, in turn, by consumptive demand of herbivores. The productive capacity of periphyton modified the nature and outcome of plant—herbivore interactions. This capacity therefore has important implications for the operation of stream ecosystems.

Effects of ice scour on the structure of sublittoral marine algal assemblages of St. Lawrence and St. Matthew Islands, Alaska

Ice scour and gouging play a major role in the distribution and abundance of macroalgae and sessile invertebrates in the northern Bering Sea. Annual ice scouring and gouging create a gradient of disturbance, at one extreme removing the entire biota from horizontal rock surfaces in shallow water ( 12 m), Laminaria spp. were less common, and Agarum cribrosum, whlch was rare in shallow water, became common. Contrary to other studies, the lower distribution of A. cribrosum is not related to grazing sea urchins, as urchins and other large herbivores are absent. The algal zone extended to a depth of ca 28 m, where sessile invertebrate cover became high. The marine benthos of St. Lawrence Island was characterized by a high cover of Laminaria spp., few understory algae, and sessile invertebrates such as barnacles, hydroids, mussels, sponges, and bryozoans, while St. Matthew Island had a high cover of Laminaria dentigera, abundant understory red algae such as Ptilota asplenioides, Cirrulicarpus gmelinii, and encrusting coralline algae, and fewer sessile invertebrates.

Effects of Herbivore Type and Density on Taxonomic Structure and Physiognomy of Algal Assemblages in Laboratory Streams

Four densities of a snail (Juga silicula) and a caddisfly (Dicosmoecus gilvipes) were introduced into separate laboratory streams, and their effects on algal biomass and community structure were monitored for 32 d. Tiles in an ungrazed control stream were covered by thick algal mats by day 32, and were composed primarily of Scenedesmus spp., Characium, and a variety of diatoms. Biomass and community structure of algal assemblages in the stream with the lowest density of snails were very similar to those in the control stream. In the other streams with snails, an inverse relationship developed between algal biomass and snail density after day 16. By day 32, the algal assemblages in the streams with high snail densities were dominated by adnate diatoms (e.g., Achnanthes lanceolata), and basal cells and short filaments of Stigeoclorium tenue. In contrast to the streams with snails, algal biomass was relatively low in all streams with caddisflies. The differences in algal biomass and structure between the streams with the lowest and highest densities of caddisflies were much smaller than those between streams with the lowest and highest densities of snails. On day 32, the taxonomic and physiognomic structure of the algal assemblages in all the streams with caddisflies resembled that in the streams with higher densities of snails. Scanning electron micrographs showed that even at the highest densities, neither snails nor caddisflies could completely remove the algal assemblage. It is concluded that grazing can substantially influence algal growth form and assemblage physiognomy in lotic ecosystems.

Effects of Irradiance on the Community Structure and Biomass of Algal Assemblages in Laboratory Streams

Studies in natural streams have implicated irradiance as a factor which has a strong influence on the dynamics of algal communities. In this study, laboratory streams were used in a replicated, experimental design to investigate whether differences in algal biomass, taxonomic structure, and physiognomy result from exposure to different irradiances. Effects of four photon flux densities (15, 50, 150, and 400 μE∙m−2∙s−1) on locally collected benthic stream algae were monitored over a 48-d period. Biomass increased in all streams during the experiment, but the streams exposed to the highest irradiance had 25 times more biomass at the end of the experiment than the channels exposed to the lowest irradiance. Although diatoms were the dominant algal class in all streams, the relative abundance of chlorophytes was much greater in streams exposed to 150 and 400 μE∙m−2∙s−1 than in channels treated with 15 and 50 μE∙m−2∙s−1 Detrended correspondence analysis indicated that the successional trajectories of assemblages exposed to low irradiances were quite distinct from those of assemblages treated with high irradiances. Observations of assemblage physiognomy by scanning electron microscopy revealed that at low irradiances, a densely packed understory of adnate diatoms, with a few overstory diatoms, covered the tile surface. At high irradiances, tiles were overlaid with thick algal mats, composed of filamentous and coenobic chlorophytes and diatoms of various growth forms (rosette, chain-forming, and solitary). The experimental results suggested that differences in biomass and community structure among the laboratory assemblages were a direct result of light energy, and that irradiance is a major factor influencing algal dynamics in lotic ecosystems.