Distribution Of Algal Biomass Research Articles

Evaluation and characterization of algal biomass applied to the development of fingermarks on glass surfaces

ABSTRACT Algae have an extensive application to bioenergetics and commercial fields. They comprise several species with distinct physiological, chemical constitution and pigmentation. Given the toxicity of current fingerprint commercial developers, marine biomass could be an alternative material to enhance latent fingerprints. In this sense, the aim of this work was to evaluate the physical and chemical properties of algal biomass (Chlorella sp., Desmarestia anceps, Laurencia dendroidea, Lessonia searlesiana and Spirulina sp.) as potential latent fingerprint developers. Results showed that algae were composed of majority biochemical constituents including carboxylic acids (1.52 ± 0.20% for D. anceps to 50.94 ± 0.83% for Chlorella sp.) and alcohols (28.39 ± 1.47% for Chlorella sp. to 83.74 ± 2.42% for L. searlesiana). Particle size of 50% of the distribution of algal biomass varied from 42.18 to 140.00 μm. Samples mostly had vibrations related to hydroxyl, carbonyl and carbon-nitrogen groups. Moreover, elementary composition showed that samples were mostly composed of metals including potassium (9.63–51.83%), calcium (5.98–40.62%) and iron (0.06–12.85%). They are known as essential mineral and have low toxicity in the human body. Regarding fingerprint enhancement, Spirulina sp. had superior results compared to other biomasses enabling satisfactory development of fingerprints.

Lidar Monitoring of Chlorophyll a During the XXIX and XXXI Italian Antarctic Expeditions

Although it is known that the Ross Sea is responsible for more than a quarter of CO2 absorption of the Southern Ocean, more information is needed to model the primary production of this key area. In particular, it is necessary to improve the characterization of the size class distribution, biomass and taxonomic composition of phytoplankton in the Ross Sea. Recently, an innovative compact lidar fluorosensor was deployed for real-time sensing of chlorophyll a, during the Ross Sea Mesoscale Experiment (RoME), conducted in the XXIX (2014) and XXXI (2016) Italian Antarctic expeditions. Furthermore, high-performance liquid chromatography (HPLC) was also performed to provide pigment analysis of in situ samples. Lidar fluorosensors are laser-induced fluorescence (LIF) instruments and have been extensively operated by the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) since the 1990s to monitor water bodies. Spectra obtained with LIF contain signatures of phytoplankton pigments, chromophoric dissolved organic matter and dispersed impurities, such as crude oils. The study of algal pigments provides not only the phytoplankton biomass—directly linked to chlorophyll a—but also its taxonomic composition through detection of several accessory pigments. Moreover, some models allow the identification of phytoplankton size classes. Lidar and HPLC mapped the spatiotemporal distribution of algal biomass and showed that during RoME, the phytoplankton assemblage structure was dominated by large-size cells (micro-phytoplankton) and the prevailing algal groups were diatoms.

Relation Significance between Hydrological Residence Time and Phytoplankton Dynamics in Lake Kinneret (Israel)

Long-term data record of Phytoplankton (Peridinium, Cyanophyta, Chlorophyta, Diatoms) dynamics in Lake Kinneret was evaluated comparatively with hydrological parameters. The hydrological features were found to be correlated with algal biomass distribution. Consequently, the best representor of the hydrological trait, Residence Time, is suggested to be a key parameter controlling algal dynamics.

High nutrient availability leads to weaker top‐down control of stream periphyton: Compensatory feeding in Ancylus fluviatilis

Abstract Benthic algal biomass and distribution in freshwater ecosystems are determined by both nutrient availability (bottom‐up control) and grazing activity by herbivores (top‐down control). Fluctuations in algal nutrient ratios may cause grazers to optimise their food intake through behavioural strategies in order to maintain a constant soft body stoichiometry. Such linkages between nutrient availability and grazing control of algal biomass are as yet poorly understood. In this study, we tested whether the stream‐dwelling freshwater gastropod Ancylus fluviatilis would increase its food consumption rate with decreasing periphyton nutrient content, a behaviour known as compensatory feeding. We performed a fully factorial microcosm experiment in which two levels of periphyton phosphorus content (low versus high) were crossed with grazer presence/absence in 12 circular flumes. After 1 week of grazing, food consumption rates were measured by determining the periphyton difference between grazed and ungrazed flumes, and the periphyton biomass variability in every flume was described with a coefficient of variation. The food consumption rate of A. fluviatilis was significantly higher in the low phosphorus compared to the high phosphorus treatment, supporting the compensatory feeding hypothesis. As a result, in the presence of grazers, periphyton biomass was significantly lower under low phosphorus availability, while biomass was not affected by nutrient enrichment in the grazer‐free flumes. Despite the strong difference in periphyton phosphorus content, A. fluviatilis soft body stoichiometry did not differ between the two nutrient treatments, suggesting strong stoichiometric homeostasis. Furthermore, the distribution of algal biomass was significantly more heterogeneous in the grazed phosphorus‐poor than in the phosphorus‐rich periphyton. Our findings suggest that nutrient enrichment may lead to a weaker top‐down control of algal biomass in stream ecosystems and to reduced spatial heterogeneity of periphyton abundance.

Antarctic pack ice algal distribution: Floe‐scale spatial variability and predictability from physical parameters

AbstractAntarctic pack ice serves as habitat for microalgae which contribute to Southern Ocean primary production and serve as important food source for pelagic herbivores. Ice algal biomass is highly patchy and remains severely undersampled by classical methods such as spatially restricted ice coring surveys. Here we provide an unprecedented view of ice algal biomass distribution, mapped (as chlorophyll a) in a 100 m by 100 m area of a Weddell Sea pack ice floe, using under‐ice irradiance measurements taken with an instrumented remotely operated vehicle. We identified significant correlations (p < 0.001) between algal biomass and concomitant in situ surface measurements of snow depth, ice thickness, and estimated sea ice freeboard levels using a statistical model. The model's explanatory power (r2 = 0.30) indicates that these parameters alone may provide a first basis for spatial prediction of ice algal biomass, but parameterization of additional determinants is needed to inform more robust upscaling efforts.

Numerical Modeling of Sediment Transport and Its Effect on Algal Biomass Distribution in Lake Pontchartrain Due to Flood Release from Bonnet Carré Spillway

In order to protect the city of New Orleans from the Mississippi River flooding, the Bonnet Carre Spillway (BCS) was constructed from 1929 to 1936 to divert flood water from the river into Lake Pontchartrain and then into the Gulf of Mexico. During the BCS opening for flood release, large amounts of freshwater, nutrients, sediment, etc. were discharged into Lake Pontchartrain, and caused a lot of environmental problems. To evaluate the environmental impacts of the flood water on lake ecosystems, a two-dimensional numerical model was developed based on CCHE2D and applied to simulate the flow circulation, sediment transport and algal biomass distribution in Lake Pontchartrain. The effect of sediment concentration on the growth of algae was considered in the model. The numerical model was calibrated using field measured data provided by USGS, and then it was validated by the BCS Opening Event in 1997. The simulated results were generally in good agreement with filed data and satellite imagery. The field observation and numerical model show that during the spillway opening for flood release, the sediment concentration is very high, which greatly restricts the growth of algae, so there is no algal bloom observed in the lake. After the closure of BCS, the sediment concentration in the lake reduces gradually, and the nutrient concentration of the lake is still high. Under these conditions, numerical results and satellite imagery showed that the chlorophyll concentration was high and algal bloom might occur.

Horizontal migration of algal patches associated with cyanobacterial blooms in an eutrophic shallow lake

Most eutrophic freshwater lakes in China have suffered from harmful cyanobacterial blooms. Phytoplankton horizontal movement plays an important role in the spatial distribution of algal biomass and algal bloom formation in eutrophic freshwater lakes. In this study, an in situ experiment was performed in Lake Zhushan, a semi-enclosed lake bay in the northwestern portion of Lake Taihu, China, to investigate the effects of hydro-meteorological factors on the horizontal movement of algal patches during August 2011, 2012 and 2014. The vertical distribution patterns of chlorophyll a under different wind regimes revealed that the critical wind speed suppressing the formation of algal patches on the water surface was ca. 3.6m/s in the lake. When wind speed was below the critical value, a significant linear correlation was found between horizontal migration velocity of algal patches and wind speed (R=0.368, n=329, p<0.001), between horizontal migration velocities of algal patches and significant wave height (R=0.107, n=154, p<0.001), and between horizontal migration velocities of algal patches and current flow rate (R=0.139, n=262, p<0.001). The horizontal movement of algal patches at the water surface was influenced by both wind and current directions, and a multiple linear regression equation (R=0.642, p=0.002) adequately described the interactive effects of wind speed, flow rate and significant wave height on horizontal migration of algal patches. These results will facilitate a better understanding of the mechanisms of phytoplankton aggregation in the upper layer and algal bloom formation in eutrophic lakes and reservoirs.

Mechanisms involved in the proliferation and distribution of phytoplankton in the Patagonian Sea, Argentina, as revealed by remote sensing studies

Studies about the mechanisms that control the dynamics of phytoplankton in terms of surface chlorophyll patches by means of remote sensing information were encouraged in the Patagonian Sea (South Atlantic Ocean). Over this sea, circulation of the Patagonian Current and Malvinas Current, originated in the Circumpolar Antarctic Current, has much to do in the distribution of algal biomass. The study confirms the existence of three ecosystems: Coastal system, Shelf waters system and Shelf break. In the former, algal blooms are detected mostly between March and August. Later, the patches move inside the shelf area, the second ecosystem, towards NNE direction from approximately August to December under the influence of the Patagonian Current to reach the Malvinas Current, which circulates along the shelf break. The third ecosystem, Malvinas system, may be divided into the Malvinas Islands, where an intense algal bloom from December to January could be verified, and the Malvinas Current that shows a high concentration of algal pigment in the same period, although evidences support the hypothesis that the latter has mainly an allochthonous character. At about 37°S–39°S and 53°W–56°W, variable position, the confluence between Malvinas Current and Brazil Current takes place. As a result, the chlorophyll that functions as a tracer indicates that the impact has the characteristics of being an elastic collision, since the Malvinas Current identity was preserved afterwards. Special events, such as the climatic conditions that prompt changes in the annual behavior of the phytoplankton distribution, are discussed.

Assessment of Spatial Distribution of Algal Biomass in a Subtropical Lake in China by Modeling of Landsat ETM+ Data Coupled with Field Data

The objective of this research is to construct an efficient way of monitoring water quality and assessing trophic state using remote sensing techniques in Qinshan Lake of Hangzhou, China. Two Landsat ETM+ images were acquired and simultaneous in situ measurements, sampling and analysis were conducted. Results of the study indicated that the ratio of ETM+1/ETM+3 was the most effective single band in estimating chlorophyll-a (Chl-a), followed by normalized ratio vegetation index (NRVI). Two multiple regression models with determination coefficients were further constructed between logarithmically transformed Chl-a and the combination of ETM+1/ETM+3, ETM+2/ETM+3, and ETM+3/ETM+4 of ten sample sites. The resulting models, Log (Chl-a)=1.65 + 0.87*(ETM+1 / ETM+3) 3.39*(ETM+2 / ETM+3) + 0.89*(ETM+3 / ETM+4), and Log (Chl-a)=2.94 1.37*(ETM+1 / ETM+3) + 0.40*(ETM+2 / ETM+3) 0.20*(ETM+3 / ETM+4), both showed strong ability to evaluate the distribution of Chl-a, with R2 of 0.72 and 0.92, respectively. Then two trophic state maps generated for Qinshan Lake using this model could identify zones with a higher potential for eutrophication, which turned out to be an appropriate method for synoptic monitoring of water quality in lakes. Similar modeling can be made for any given subtropical lake, to provide rapid and long term assessment of water quality and also useful information for decision making.

Spatial distribution of phytoplankton cells in small elongated lakes subject to weak diurnal wind forcing

The horizontal distribution of phytoplankton in a medium-size canyon type reservoir, forced by weak winds of 3–4 ms−1, is largely driven by the interaction of the large-scale circulation and processes regulating the vertical distribution of algal cells in the water column. These drivers, in turn, are subject to diurnal variations, making our understanding of the horizontal distribution of phytoplankton a challenging task. A three-dimensional physical-–ecological model is used to understand the spatial distribution of algae and the role of diurnal variations in the physical–biological drivers. The model was used to demonstrate that the large-scale circulation induced during the day is more efficient generating patchiness than the circulation existing at night, when convectively driven turbulence homogenizes the upper layers. Different spatial distributions develop for different populations and under different forcing scenarios, characterized in terms of the directionality of wind forcing, wind magnitude and the lags between winds and diel heat fluxes. The time scales needed so that algal biomass in the surface exhibit significant longitudinal gradients—T P —will vary depending on the algal group, and its ability to regulate its vertical position. These scales are shorter for those species that either migrate actively in the water column or exhibit positive buoyancy (Cryptophytes or Positive buoyant algae). In El Gergal, these scales T P are on the order of a few days. Synoptic changes in the meteorological forcing, like the passing of a front, could potentially change the longitudinal distribution of algal biomass if they persist for periods of time longer than T P .

Chlorophyll a in Antarctic sea ice from historical ice core data

Sea ice core chlorophyll a data are used to describe the seasonal, regional and vertical distribution of algal biomass in Southern Ocean pack ice. The Antarctic Sea Ice Processes and Climate – Biology (ASPeCt – Bio) circumpolar dataset consists of 1300 ice cores collected during 32 cruises over a period of 25 years. The analyses show that integrated sea ice chlorophyll apeaks in early spring and late austral summer, which is consistent with theories on light and nutrient limitation. The results indicate that on a circum‐Antarctic scale, surface, internal and bottom sea ice layers contribute equally to integrated biomass, but vertical distribution shows distinct differences among six regions around the continent. The vertical distribution of sea ice algal biomass depends on sea ice thickness, with surface communities most commonly associated with thin ice (&lt;0.4 m), and ice of moderate thickness (0.4–1.0 m) having the highest probability of forming bottom communities.

Small-Scale Vertical Distribution of Algae and Structure of Lichen Soil Crusts

Although many studies have been conducted on the ecological functions and ecophysiological characteristics of lichen soil crusts (LSCs), no explanation of these results has been provided based on crust structures. Using algae soil crusts (ASCs) as comparison, this work studied the small-scale vertical distribution of algal biomass and stratification in two types of LSCs, by combining the binocular stereomicroscope observations, microscope observations, plate cultures, chlorophyll analysis, and polysaccharides analysis. The results showed an obvious difference in the proportion but not the composition of the algae species between the ASCs and the two LSCs. Approximately 60% and 80% of the total algal biomass were concentrated in the top 1mm of the soil profile and thalli in the ASCs and LSCs, respectively. This implies that symbiotic algae are the dominant species and primary organic carbon producers in LSCs, and the algal biomass decreased with the depth in both the ASCs and LSCs. The small-scale vertical distributions of the crustal algal biomass and polysaccharides were characterized by obvious successional stage, whereas these were unrelated to the crust type within the successional stage. Additionally, a large amount of fungi, which were always piercing the entire crusts, were observed in the LSCs, but only occasionally in the ASCs. These special structures are purported to cause the LSCs to achieve specific ecological functions, such as higher carbon fixation and greater compressive strength. High biomass, large living space, and advantageous resource utilization privilege suggest that the lichen association is mutualistic and the direction from ASCs to LSCs is developmental.

Priming the productivity pump: flood pulse driven trends in suspended algal biomass distribution across a restored floodplain

Summary1. Chlorophyll a (Chl a) distribution across a 0.36 km2 restored floodplain (Cosumnes River, California) was analysed throughout the winter and spring flood season from January to June 2005. In addition, high temporal‐resolution Chl a measurements were made in situ with field fluorometers in the floodplain and adjacent channel.2. The primary objectives were to characterise suspended algal biomass distribution across the floodplain at various degrees of connection with the channel and to correlate Chl a concentration and distribution with physical and chemical gradients across the floodplain.3. Our analysis indicates that periodic connection and disconnection of the floodplain with the channel is vital to the functioning of the floodplain as a source of concentrated suspended algal biomass for downstream aquatic ecosystems.4. Peak Chl a levels on the floodplain occurred during disconnection, reaching levels as high as 25 μg L−1. Chl a distribution across the floodplain was controlled by residence time and local physical/biological conditions, the latter of which were primarily a function of water depth.5. During connection, the primary pond on the floodplain exhibited low Chl a (mean = 3.4 μg L−1) and the shallow littoral zones had elevated concentrations (mean = 4.6 μg L−1); during disconnection, shallow zone Chl a increased (mean = 12.4 μg L−1), but the pond experienced the greatest algal growth (mean = 14.7 μg L−1).6. Storm‐induced floodwaters entering the floodplain not only displaced antecedent floodplain waters, but also redistributed floodplain resources, creating complex mixing dynamics between parcels of water with distinct chemistries. Incomplete replacement of antecedent floodplain waters led to localised hypoxia in non‐flushed areas.7. The degree of complexity revealed in this analysis makes clear the need for high‐resolution spatial and temporal studies such as this to begin to understand the functioning of dynamic and heterogeneous floodplain ecosystems.

Benthic responses to groundwater–surface water exchange in 2 alluvial rivers in northwestern Montana

AbstractWe tested the hypotheses that groundwater–surface water exchange regimes affect spatial distribution of algal biomass and Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa in main-channel riffle habitats of 2 northwestern Montana rivers flowing through alluvial flood plains. We used a stratified random design to sample riffles with contrasting groundwater–surface water exchange regimes, but with similar bedform, current velocity, and substrata grain size. We examined general patterns of exchange between river channel and hyporheic zone waters by measuring vertical hydraulic gradients (VHG) and hydraulic conductivities using mini-piezometers. Riffles near the upstream limit of each flood plain were characterized by strong, hyporheic recharge (−VHG, downwelling), whereas riffles throughout the lower half of each flood plain were characterized by weak, dispersed hyporheic discharge (+VHG, upwelling). There were no differences in mean seston concentrations between riffles on either flood plain wit...

The altitudinal distribution of snow algae on an Alaska glacier (Gulkana Glacier in the Alaska Range)

AbstractThe altitudinal distribution of a snow algal community was investigated on an Alaska glacier (Gulkana Glacier in the Alaska Range) from 1270 to 1770 m a.s.l.. Seven species of snow and ice algae (Chlorophyta and cyanobacteria) were observed on the glacier surface. These species wereChlamydomonas nivalis,Mesotaenium berggrenii,Ancylonema nordenskioldii,Cylindrocystis brébissonii,Raphidonemasp., and two Oscillatoriaceae cyanobacteria. The altitudinal distribution of snow algae was different among the species:Cd. nivaliswas distributed on the middle to upper area,M. berggrenii;A. nordenskioldii, and one Oscillatoriaceae cyanobacterium on the middle to lower area;Raphidonemasp. on the middle area; andCyl. brébissoniiand one Oscillatoriaceae cyanobacterium on the lower area. The total cell concentration and the cell volume biomass of the snow algae ranged from 4·4 × 103to 9·9 × 105cells ml−1and from 33 to 2211 µl m−2respectively. The cell volume biomass changed with altitude; the biomass increased with altitude below 1600 m a.s.l., and decreased above 1600 m a.s.l. The community structure showed thatA. nordenskioldiidominated on the lower part of the glacier, and thatCd. nivalisdominated on the upper part. The species diversity was relatively high at the lowest and middle sites. The pH was 4·7 to 5·3 for snow and 4·9 to 5·7 for ice on the glacier. The altitudinal distribution of snow algae is discussed in terms of the physical and chemical condition of the glacier surface, and is compared with that on a Himalayan glacier. A larger biomass in the snow area on the Alaska glacier than that of the Himalayan glacier is likely due to less frequent snow cover in summer in Alaska. Small amounts of filamentous cyanobacteria on the Alaska glacier may allow washouts of unicellular green algae by running melt water and may cause a different pattern of altitudinal distribution of algal biomass on the ice area from the Himalayan glacier. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Spatial distribution of phytoplankton in the Gulf of Riga during spring and summer stages

Distribution patterns of chlorophyll a, phytoplankton species and biomass were studied in the Gulf of Riga, one of the most eutrophicated areas of the Baltic Sea. Quasi-synoptic measurements were carried out during four seasonal stages (spring bloom 1995, early-summer stage 1994, cyanobacterial bloom 1994, and late summer stage 1993). For each stage, common factor analysis was used to simplify the highly correlated patterns of nutrients, salinity, temperature and the depth of mixed layer. Obtained latent variables were used to explain spatial distribution of phytoplankton. Generally, the distribution of phytoplankton variables followed closely the patterns of nutrient rich fresh water. During the spring bloom in 1995, the spatial structures of phytoplankton biomass (4–27 mg l −1) and chlorophyll a (13–50 μg l −1) were well explained ( r=0.68 and 0.69, respectively, p<0.01) by common factor loaded by inorganic N, total N and P, salinity and the depth of mixed layer. The early-summer stage in 1994 was characterized by homogenous distribution of algal biomass and low concentrations of inorganic N and P. At this stage, the decreasing amount of total N and P from the southern Gulf of Riga towards the northern part of the Gulf was accompanied with the decrease of chlorophyll a concentrations and the increase of relative importance of picoplankton (algae <2 μm). Intense bloom of cyanobacteria ( Aphanizomenon flos-aquae (L.) Ralfs) took place in 1994 after warming up of the surface layer. The abundance of A. flos-aquae was related to the common structure of N, P and fresh water and the bloom was most dense (up to 6.6 mg l −1) in the southern Gulf. At the same time, the contribution of picoplankton from total chlorophyll a was high (up to 60%) in the northern Gulf affected less by the nutrient load. The late summer phase in 1993 was characterized by high heterotrophic activities and the phytoplankton community was dominated by cryptomonads (on the average 50% of total biomass). At that stage the distribution of phytoplankton was obviously governed by biological interactions and the multivariate methods were not especially successful to explain the spatial distribution of the main components of phytoplankton community. Excluding the late summer phase, the statistical methods used in this study revealed well the relationships between phytoplankton variables and nutrient rich fresh water. It is evident that nutrient load from the River Daugava is a reason for higher phytoplankton biomass in the southern Gulf of Riga, especially during bloom occasions.

Benthic algal biomass in an unshaded first-order lowland stream: distribution and regulation

The vertical distribution of algal biomass in the bed sediment and the seasonal development of benthic algae on stones and fine-grained sediments were studied in a small unshaded stream. In addition, field experiments were conducted on the role of irradiance and phosphorus in regulating algal biomass. We found that algal biomass was high at a sediment depth of ten centimetres. Comparison of studies on algal biomass where different depths of the sediment are used should therefore be made with caution. Substrata-dependent differences in algal biomass development were substantial. While algal biomass development on stones was controlled by macroinvertebrate grazing, that on the fine-grained sediment followed the dynamics of incident irradiance, but was attenuated by sediment rebedding. Because of the high grazing pressure on algal biomass on stony substrata, no significant response to phosphorus enrichment was attained. In contrast, algal biomass development on fine-grained sediments was phosphorus-limited. Heavy shading of the fine-grained sediments did not significantly affect algal biomass development, thus suggesting that phosphorus limitation prevents algae from fully utilizing the light resource in this stream.

Spatial and temporal distribution of algal biomass in a large, subtropical lake

Epipelon, epiphyton, and phytoplankton biomass were measured quarterly for one year from 20 stations located throughout Lake Okeechobee, a large, subtropical lake located in south-central Florida, USA. Seasonal variability was evident in all algal components. The water column accounted for the majority of chlorophyll-a (>90 %) in December, March, and September, but accounted for only 61.1% of total chlorophyll-a in June. Conservative estimates of total chlorophyll-a biomass ranged from 1300 to 36,600kg kg for attached algae, and from 41,770 to 206,600kg kg for phytoplankton. Overall, the greatest epipelon biomass was measured in June, the greatest epiphyton biomass was measured in March, and the greatest phytoplankton biomass was measured in December in the pelagic region and in June in the littoral region. Stations deep within the littoral zone had greater epipelic and epiphytic biomass than those located along the fringe zone between the pelagic and littoral zones. Principal components analyses revealed that stations located within the marsh formed the most distinct groupings. Light-related factors were important in separating stations along axis 1, whereas water chemistry helped separate stations along axis 2. Overall, these results suggest that epipelon, epiphyton, and phytoplankton communities are responding differently to the forcing functions in this large, subtropical lake. Management decisions influencing lake level may affect these communities differentially, which in turn, may influence ecosystem processes.

Habitat Heterogeneity and Algal-Grazer Interactions in Streams: Explorations with a Spatially Explicit Model

We present results from a spatially explicit model that makes predictions about how physical habitat heterogeneity mediates algal-grazer interactions. We simulated the responses of mean algal biomass and variability in the distribution of algae to 2 levels of physical habitat heterogeneity (1 vs. 15 boulders) under 2 velocity regimes (slow vs. fast riffles). Algal growth occurred locally in a spatially explicit grid where local current was specified for each of the 60 × 30 grid cells. Grazer movement and foraging were simulated using an individual-based model, in which the direction and rate of movement of individual grazers were determined at each time step in relation to local current and algal standing crop. Model parameterization was based on field observations, laboratory experiments, and literature review. In each of the 4 simulations, algae were first allowed to grow in the absence of grazing to near carrying capacity. Neither habitat heterogeneity nor riffle velocity regime had a significant effect on the mean biomass or spatial variability of algae. Next, algae were exposed to 3 simulated grazer densities and the subsequent effects on algal biomass and patchiness were determined after prolonged contact. We performed replicate simulation runs to allow statistical inferences to be drawn about differential responses of algae to the various "treatments." As expected, mean algal biomass declined in proportion to grazer density. Higher habitat heterogeneity resulted in reduced algal biomass for low and intermediate grazer densities, but only in the slow riffle. Otherwise, the effects of specified grazer densities did not vary between slow and fast riffles. Variability in algal biomass distribution was measured in 2 ways. First, a spatially explicit index of relative patchiness showed a gradual increase under low grazer density but a transient peak followed by decline under moderate and intense grazing pressure, a pattern observed for both slow and fast current regimes. In the slow riffle alone, heterogeneity and grazer density interacted to influence algal patchiness. A second, non-spatially explicit index of variability in algal biomass, the coefficient of variation, increased as grazer density increased regardless of current regime. Higher variability in the distribution of algal biomass tended to be associated with greater habitat heterogeneity, especially in the slow riffle. The simulations suggested that spatial heterogeneity and ambient current velocity regime mediate algal-grazer interactions. The extent to which grazers crop biomass and create spatial variability in the distribution of algal biomass may depend not only on grazer density but also on the interaction between grazer density, physical habitat heterogeneity, and stream velocity. Given this complexity, empirical approaches to understanding algal-grazer interactions in spatially and temporally heterogeneous stream systems are likely to be greatly augmented by modelling, which allows different environmental conditions and grazer foraging behaviors to be simulated.

Effects of Nutrients and Planktivorous Fish on the Phytoplankton of Shallow and Deep Aquatic Systems

The response of phytoplankton to nutrient (N, P) and planktivorous fish additions (Phoxinus sp.) was compared between two sets of enclosures (8 m in diameter) installed at different depths in a dimictic temperate lake. One set of eight enclosures was located in the deep part of the lake (12 m) where water thermally stratifies during the summer. The other set of eight enclosures was located in a shallow area of the lake (3.5 m) where the water does not thermally stratify. During the summer months, phytoplankton cell number and biomass increased significantly with both nutrient additions (P = 0.004) and the presence of planktivorous fish (P = 0.011) in the deep, thermally stratified enclosures. On average, total algal biomass in the nutrient and fish treatments was 2.6 times higher than in the nutrient—only treatments. In contrast, in the shallow enclosures, total phytoplankton biomass increased with nutrient additions (P = 0.04) but was not significantly affected by the addition of fish (P = 0.34). However, fish additions increased the number of algal cells in both the shallow and deep enclosures, independently of nutrients, because of an increase in small cells: the proportion of nanoplankton cells was significantly greater in the presence of fish. Fertilization increased the large nanoplankton fraction (10—20 μm) in the deep enclosures, but had no significant effect on the size distribution of algal biomass in the shallow enclosures. In general, large (&gt; 64 μ m) cells were a minor component of the phytoplankton biomass in all treatments. Fertilization stimulated the growth of Chlorophyta in the deep (P = 0.018) and in the shallow (P = 0.012) enclosures. Cyanobacteria were conspicuous in the fertilized shallow enclosures, but the effect on total biomass was not statistically significant. The presence of fish stimulated a significant increase in Cryptophyta at both depths and Pyrrophyta were more common. Nutrients and planktivorous fish had different effects on phytoplankton depending on the thermal regime. In terms of total algal biomass there was no significant difference between deep and shallow enclosures within a given treatment, but depth did affect the size distribution of algal biomass among treatments. The effects of fish were more evident with community structure variables (size or taxonomic distribution) than with coarser variables such as algal biomass.