THE IMPORTANCE OF SHALLOW SEDIMENTS IN THE RECRUITMENT OF ANABAENA AND APHANIZOMENON (CYANOPHYCEAE)1

Recruitment of Anabaena and Aphanizomenon from the sediments to the water column was investigated in shallow (1–2 m) and deep (6–7 m) areas of Lake Limmaren, central Sweden. Recruitment traps attached to the bottom were sampled weekly throughout the summer season (June through September). A comparison between the two sites shows that the largest part of the recruited cells originated from the shallow site, although recruitment occurred at all depths in the lake. There were also differences between the species, regarding the site as well as the timing of the recruitment. The contribution of the inoculum to the pelagic population was calculated to vary between 0.003% and 0.05% for the different species. From these results we conclude that shallow sediments are more important than deep ones for the recruitment and that the inoculum in Lake Limmaren is small but may still be an important factor in the population dynamics.     

RECRUITMENT OF BENTHIC MICROCYSTIS (CYANOPHYCEAE) TO THE WATER COLUMN: INTERNAL BUOYANCY CHANGES OR RESUSPENSION?1

In some lakes, large amounts of the potentially toxic cyanobacterium Microcystis overwinter in the sediment. This overwintering population might inoculate the water column in spring and promote the development of dense surface blooms of Microcystis during summer. In the Dutch Lake Volkerak, we found photochemically active Microcystis colonies in the sediment throughout the year. The most vital colonies originated from shallow sediments within the euphotic zone. We investigated whether recruitment of Microcystis colonies from the sediment to the water column was an active process, through production of gas vesicles or respiration of carbohydrate ballast. We calculated net buoyancy, as an indication of relative density, using the amounts and densities of the major cell constituents (carbohydrates, proteins, and gas vesicles). Carbohydrate content of benthic Microcystis cells was very low throughout the year. Buoyancy changes of benthic Microcystis were mostly a result of changes in gas vesicle volume. Before the summer bloom, net buoyancy and the amount of buoyant colonies in the sediment did not change. Therefore, recruitment of Microcystis from the sediment does not seem to be an active process regulated by internal buoyancy changes. Instead, our observations indicate that attachment of sediment particles to colonies plays an important part in the buoyancy state of benthic colonies. Therefore, we suggest that recruitment of Microcystis is more likely a passive process resulting from resuspension by wind‐induced mixing or bioturbation. Consequently, shallow areas of the lake probably play a more important role in recruitment of benthic Microcystis than deep areas.     

Seasonal variations of Microcystis populations in sediments of Lake Biwa, Japan

Seasonal variations of colony numbers of Microcystis aeruginosa(Kütz.) Kütz. and M. wesenbergii(Komárek) Komárek in N. V. Kondrat. in sediments of Lake Biwa were investigated over a period of 1 year. At two stations located in the shallow South Basin of Lake Biwa (ca. 4 m water depth), the colony number of Microcystisfluctuated seasonally. The number had a tendency to gradually decrease from winter to early summer, while it increased through mid-summer and autumn. Since the Microcystispopulation in sediment was rather small, intensive growth and accumulation in the water column should be important for the formation of Microcystisblooms in Lake Biwa. Microcystiscolonies in the sediment samples after June were observed to be floating in a counting chamber under a microscope. The observation suggests that the recruitment of Microcystis colonies into the water column mostly occurs in early summer. The number of Microcystiscolonies in the deep North Basin of Lake Biwa (70 – 90 m water depth) was larger than in the South Basin. Because the seasonal variation of colony numbers was not observed in the North Basin, and Microcystiscells do not have gas vesicles, these colonies will not return into the water column. The colonies isolated from the sediment of the North Basin were able to grow in cultured conditions, in the same way as those from the sediment of the South Basin. Therefore, Microcystiscolonies may survive for a long time under stable conditions of low temperature (ca. 8 °C) and darkness, in the sediment of the deep North Basin, accumulating gradually each year.     

Benthic–pelagic coupling in the population dynamics of the harmful cyanobacterium Microcystis

1. In eutrophic lakes, large amounts of the cyanobacterium Microcystis may overwinter in the sediment and re‐inoculate the water column in spring. 2. We monitored changes in pelagic and benthic populations of Microcystis in Lake Volkerak, The Netherlands. In addition, sedimentation rates and the rate of recruitment from the sediment were measured using traps. These data were used to model the coupling between the benthic and pelagic populations and to calculate the contribution of overwintering benthic and pelagic populations to the magnitude of the pelagic summer bloom. 3. Changes in the benthic Microcystis population showed a time lag of 3–14 weeks compared with the pelagic population. This time lag increased with lake depth. The largest amount of benthic Microcystis was found in the deepest parts of the lake. These observations suggest horizontal transport of sedimented Microcystis from shallow to deep parts of the lake. 4. Recruitment from and sedimentation to the sediment occurred throughout the year, with highest recruitment and sedimentation rates during summer. Model simulations indicate that the absence of benthic recruitment would reduce the summer bloom by 50%. 5. In spring, the total pelagic population was three to six times smaller than the total benthic population. Yet, model simulations predict that the absence of this small overwintering pelagic population would reduce the summer bloom by more than 64%. 6. Reduction of the overwintering pelagic populations, for instance by flushing, may be a useful management strategy to suppress or at least delay summer blooms of Microcystis.     

Cyanobacterial toxicity and migration in a mesotrophic lake in western Washington, USA

In fall 1997, the toxic cyanobacterium Microcystis aeruginosa was documented in Lake Sammamish (western Washington, U.S.A.) for the first time. Cyanobacterial activity and environmental conditions that may promote toxic cyanobacteria were investigated during summer and fall 1999. Development of toxic Microcystis was hypothesized to be due to runoff of nutrients from the watershed (external loading hypothesis) or from vertical migration of dormant cyanobacteria from the nutrient-rich sediments into the water column (cyanobacterial migration hypothesis). Microcystins were detected using an enzyme-linked immunosorbent assay during late August and early September 1999 despite low cyanobacterial abundance. Microcystin concentrations ranged between 0.19–3.8 g l^–1 throughout the lake and at all depths with the exception of the boat launch where concentrations reached 43 g l^–1. Comparison of the conditions associated with the toxic episodes in 1997 and 1999 indicate that Microcystis is associated with a stable water column, increased surface total phosphorus concentrations (> 10 g l^–1), surface temperatures greater than 22°C, high total nitrogen to phosphorus ratios (> 30), and increased water column transparency (up to 5.5 m). Migration of the cyanobacteria, Microcystis and Anabaena, occurred in both the deep and shallow portions of the lake. Microcystis dominated (89–99%) the migrating cyanobacteria with greater migration from the shallow station. External loading of nutrients due to the large rainfall preceding the 1997 toxic episode may have provided the nutrients needed to fuel that bloom. However, toxic Microcystis occurred in 1999 despite the lack of rain and subsequent external runoff. The migration of Microcystis from the nutrient-rich sediments may have been the inoculum for the toxic population detected in 1999.     

RECRUITMENT AND PELAGIC GROWTH OF GLOEOTRICHIA ECHINULATA (CYANOPHYCEAE) IN LAKE ERKEN1

Different parameters in the life cycle of the colony forming cyanobacterium Gloeotrichia echinulata (J.E. Smith) Richter was evaluated in Lake Erken, Sweden. Recruitment of colonies from the sediments and pelagic abundance were measured during 2 years. These data were then used in a model to evaluate and estimate parameters of the life cycle. In our study, recruitment alone only contributed to a small part (<5%) of the maximum G. echinulata abundance that occurred during late summer. However, recruitment from shallow sediments forms the important seed for the pelagic population. Together with measured rates of migration from the sediment, variations in either pelagic colony division rate or pelagic residence time could explain variations in the measured abundance of G. echinulata in situ.     

ROLE OF RECRUITMENT IN STRUCTURING BEDS OF SARGASSUM SPP. (PHAEOPHYTA) AT ROTTNEST ISLAND,WESTERN AUSTRALIA1

The importance of annual recruitment to the structure of adult stands of Sargassum was determined for a mixed species Sargassum bed at Rottnest Island, Western Australia. The morphologically similar species Sargassum spinuligerum Sonder, S. distichum Sonder, and S. podacanthum Sonder grew together in the shallow subtidal (6 m). Positive species determinations were only possible when thalli were reproductive, so recruits, bases, and vegetative annuals for all species were grouped together. Densities of recruits, perennial bases, vegetative annuals, and reproductive annuals were determined at monthly intervals from 20 randomly placed 0.25-m^?2 quadrats. Recruitment and mortality for recruits and adults were further determined at three monthly intervals from 6-×-1-m^?2 permanent quadrats. The density of adults varied little with season (between 32 and 58 m^?2). Growth of annuals was initiated in April, thalli became reproductive by late August–early September, and senescence occurred in December–January. Density of recruits was highly variable (1.6–210 individuals-m^?2) and peaked seasonally during late summer (January–February) and then declined rapidly. Adults showed a complete turnover of thalli in the bed over 25–27 months. Adult mortality was compensated by annual recruitment from propagules (43%) and vegetative regeneration from fragments of holdfasts left on the reef (57%). A seasonal pattern in survivorship was observed for adults that grew from recruits with higher initial numbers and lower mortalities for August and November cohorts. Little seasonally was observed in survivorship of adults that grew vegetatively from remnant crusts. Although initial cohort sizes were smaller for adults grown from recruits than from remnant crusts, mortality was lower, resulting in similar contributions to adult density from both recruits and remnant crusts. Recruitment from propagules and vegetative regeneration played an important role in buffering the adult stand from high rates of mortality and reducing seasonal variation in adult density and contributed to the persistence and seasonal structure of Sargassum beds at Rottnest Island.     

INVOLVEMENT OF MICROCYSTINS AND COLONY SIZE IN THE BENTHIC RECRUITMENT OF THE CYANOBACTERIUM MICROCYSTIS (CYANOPHYCEAE)1

The benthic recruitment of Microcystis was simulated in vitro in order to characterize the colonies of Microcystis recruited and to study the impact of intracellular and extracellular microcystins (MCs), and the influence of colony size on the recruitment process. We observed recruitment dynamics consisting of a lag phase followed by a peak and then a return to low recruitment rates, mainly controlled by passive resuspension throughout the experiment, and by physiological processes during the recruitment peak. Ninety‐seven percent of the Microcystis colonies recruited were <160 μm in maximum length, and their cells contained much greater amounts of MCs (0.26 ± 0.14 pg eq microcystin leucine‐arginine variant [MC‐LR] · cell^?1) than those in benthic colonies (0.021 ± 0.004 pg eq MC‐LR · cell^?1). The MC content of recruited Microcystis varied significantly over time and was not related to changes in the proportion of potentially toxic genotypes, determined using real‐time PCR. On the other hand, the changes in MC content in the potentially toxic Microcystis recruited were closely and negatively correlated with recruitment dynamics; the lowest MC contents corresponded to high recruitment rates, and the highest MC contents corresponded to low recruitment rates. Thus, depending on temperature and light conditions, these variations are thought to result from the selection of various subpopulations from among the smallest and the most toxic of the initial benthic population. Adding purified MC‐LR to experimental treatments led to a decreased recruitment of Microcystis and more specifically of mcyB genotypes.     

AN “ENVIRO‐INFORMATIC” ASSESSMENT OF SAGINAW BAY (LAKE HURON,USA) PHYTOPLANKTON: DATA‐DRIVEN CHARACTERIZATION AND MODELING OF MICROCYSTIS (CYANOPHYTA)1

Phytoplankton and Microcystis aeruginosa (Kütz.) Kütz. biovolumes were characterized and modeled, respectively, with regard to hydrological and meteorological variables during zebra mussel invasion in Saginaw Bay (1990–1996). Total phytoplankton and Microcystis biomass within the inner bay were one and one‐half and six times greater, respectively, than those of the outer bay. Following mussel invasion, mean total biomass in the inner bay decreased 84% but then returned to its approximate initial value. Microcystis was not present in the bay during 1990 and 1991 and thereafter occurred at/in 52% of sample sites/dates with the greatest biomass occurring in 1994–1996 and within months having water temperatures >19°C. With an overall relative biomass of 0.03 ± 0.01 (mean + SE), Microcystis had, at best, a marginal impact upon holistic compositional dynamics. Dynamics of the centric diatom Cyclotella ocellata Pant. and large pennate diatoms dominated compositional dissimilarities both inter‐ and intra‐annually. The environmental variables that corresponded with phytoplankton distributions were similar for the inner and outer bays, and together identified physical forcing and biotic utilization of nutrients as determinants of system‐level biomass patterns. Nonparametric models explained 70%–85% of the variability in Microcystis biovolumes and identified maximal biomass to occur at total phosphorus (TP) concentrations ranging from 40 to 45 μg · L^?1. From isometric projections depicting modeled Microcystis/environmental interactions, a TP concentration of <30 μg · L^?1 was identified as a desirable contemporary “target” for management efforts to ameliorate bloom potentials throughout mussel‐impacted bay waters.     

Recruitment of Total Phytoplankton,Chlorophytes and Cyanobacteria from Lake Sediments Recorded by Photosynthetic Pigments in a Large,Shallow Lake (Lake Taihu,China)

Recruitment of total phytoplankton, chlorophytes and cyanobacteria from lake sediments to the water column was studied using photosynthetic pigments at one site (1.5 m) in Lake Taihu, a large shallow lake in China. Samples were taken weekly from the migration traps installed on the bottom from March to May 2004. Abundance of total phytoplankton, chlorophytes and cyanobacteria were represented by Chlorophyll (Chl) a, b, and phycocyanin (PC), respectively. Over the three months, total phytoplankton, chlorophytes, and cyanobacteria corresponding to 48.9%, 68.9% and 316.2% of their initial concentrations in surface sediments were recruited in Lake Taihu. However, compared with their increase in pelagic abundance over the same period, the recruitment accounted for a rather small inoculum. Accompanying the recruitment, total phytoplankton and chlorophytes declined and cyanobacteria increased in the upper 0–2 cm sediments; colonies of Microcystis aeruginosa in the water column enlarged from small size with several cells to large colonies with hundreds of cells. Thus, overwintering and subsequent growth renewal of pelagic phytoplankton merits further study and comparison with benthic survival and recruitment. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microbial activity and phosphorus dynamics in eutrophic lake sediments enriched with Microcystis colonies

1. Sediments from hypereutrophic Lake Vallentunasjön were enriched with Microcystis colonies from the lake water, thereby simulating the conditions after the autumn sedimentation. Release of phosphorus to the overlying lake water was followed during 2–3 weeks in the laboratory. X-ray microanalysis of individual Microcystis and bacterial cells, and chemical phosphorus fractionation, were used to assess the phosphorus pool size in different fractions of the sediment. 2. Benthic Microcystis colonies, most of these having survived within the sediment for 1 year or more, were less susceptible to decomposition, and the specific growth rate of bacteria in their mucilage was lower than for other sediment bacteria. 3. Pelagic Microcystis colonies from late August were resistant to decomposition, when placed on the sediments. When Microcystis colonies from a declining pelagic population in October were added to the sediments, however, a substantial fraction of these colonies was decomposed. The specific growth rate of mucilage bacteria was five times higher than for other sediment bacteria. 4. Release of molybdate-reactive phosphorus to the overlying lake water was larger from sediment cores enriched with Microcystis colonies than from control cores. Chemical phosphorus fractionation showed a decrease in organic-bound phosphorus (residual P). 5. X-ray microanalysis showed that the phosphorus bound in Microcystis cells decreased by -0.300 mg g^?1 DW in the October experiment, due both to a decrease in biomass (i.e. mineralization) and to a decrease in phosphorus content in the remaining cells. Heterotrophic bacteria increased their cellular concentration of phosphorus. The net release of phosphorus from the Microcystis and bacterial pools corresponded to 74% of the decrease of organic-bound phosphorus in the chemical phosphorus fractionation, and to 65% of the decrease of total phosphorus in the upper 0–1 cm of the sediment. 6. Benthic bacteria and cyanobacteria may thus contribute significantly to changes in phosphorus content and turnover of the sediment by changes in their biomass, turnover rate and cellular phosphorus content.     

Factors regulating recruitment from the sediment to the water column in the bloom-forming cyanobacterium Gloeotrichia echinulata

1. The influence of light, temperature, sediment mixing and sediment origin (water depth) on the recruitment of the cyanobacterium Gloeotrichia echinulata was examined in the laboratory. 2. Light and temperature were the most important factors initiating germination in G. echinulata. 3. The extent of germination (recruited biovolume) was mainly regulated by temperature and sediment mixing. Furthermore, sediment mixing significantly enhanced the frequency of observed heterocysts and colonies. 4. Despite the fact that the deep and shallow sediments contained a similar number of akinete colonies, the highest recruitment occurred from shallow sediments, indicating that akinetes from shallow sediments have a higher viability than those from deeper parts of the lake. 5. Our results support the hypothesis that shallow sediments are more important than profundal sediments for the recruitment of G. echinulata to the pelagic zone.     

Toxic Microcystis (cyanobacteria) inhibit recruitment of the bloom‐enhancing invasive bivalve Limnoperna fortunei

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Sediment-to-water blue-green algal recruitment in response to alum and environmental factors

The sediment-to-water recruitment of blue-green algae was investigated in a shallow lake following treatment with aluminum sulfate and sodium aluminate to control sediment phosphorus (P) release. A comparison of results from two summers each before and after treatment indicates that the treatment did not universally impact the recruitment of either sporulating or non-sporulating forms of blue-green algae. Blooms of Anabaena, Aphanizomenon, and Coelosphaerium resulted predominantly from growth in the water column following strong recruitment episodes lasting up to two weeks, while Microcystis populations were relatively insensitive to periodically high inputs from recruitment. The development of planktonic populations of Gloeotrichia echinulata, by contrast, were largely dependent on sustained recruitment in response to adequate light and temperature regimes at the sediment surface.The cellular P content of recruited G. echinulata colonies was unaffected by the accumulation of aluminum floc to the lake sediments. Both G. echinulata and C. naegelianum showed elevated levels of cellular P in newly recruited colonies as compared to planktonic colonies, indicating P transport from the sediments to the water column. Total P translocation by blue-green algae was negligible in the absence of a substantial recruitment of G. echinulata. The recruitment of G. echinulata, and hence the magnitude of P translocation, was therefore more responsive to environmental conditions prevalent at the sediments than to direct effects of the treatment itself.     

COMPOSITION,DISTRIBUTION, AND ABUNDANCE OF DEEP‐WATER (>30 m) MACROALGAE IN CENTRAL CALIFORNIA1

The deep‐water macroalgal assemblage was described at 14 sites off the central California coast during 1999 and 2000 from SCUBA and remotely operated vehicle sampling. The stipitate kelp Pleurophycus gardneri Setchell & Gardner, previously thought to be rare in the region, was abundant from 30 to 45 m, forming kelp beds below the well‐known giant kelp forests. Macroalgae typically formed three broadly overlapping zones usually characterized by one or a few visually dominant taxa: 1) the upper “Pleurophycus zone” (30–45 m) of stipitate kelps and Desmarestia spp. with a high percent cover of corallines, low cover of uncalcified red algae, and rare green algae; 2) a middle “Maripelta zone” (40–55 m) with other uncalcified red algae and infrequent corallines and green algae; and 3) a zone (55–75 m) of infrequent patches of nongeniculate coralline algae. The green alga Palmophyllum umbracola Nelson & Ryan, not previously reported from the Northeast Pacific, was found over the entire geographical range sampled from 35 to 54 m. Year‐round profiles of water column irradiance revealed unexpectedly clear water with an average K₀ of 0.106·m ^{? 1} Received 18 January 2002. Accepted 16 December 2002. . The low percent surface irradiance found at the average lower macroalgal depth limits in this study (0.56% for brown algae, 0.12% for uncalcified red algae, and 0.01% for nongeniculate coralline algae) and lack of large grazers suggest that light controls the lower distributional limits. The ubiquitous distribution, perennial nature, and similar lower depth limits of deep‐water macroalgal assemblages at all sites suggest that these assemblages are a common persistent part of the benthic biota in this region.     

IMPROVED PROCEDURES FOR THE CLONING AND PURIFICATION OF MICROCYSTIS CULTURES (CYANOPHYTA)1

A cloned axenic culture of Microcystis Kützing was obtained by combining two procedures: a) the disaggregation of multicellular Microcystis colonies by dilution into deionized water, and b) the selective growth of Microcystis in agar media containing Na₂S, which inhibited or killed the associated contaminants. Microcystis growth was stimulated by 0.3–1 mM Na₂SO₃, but not by 0.1–33 mM Na₂SO₄. Although Microcystis cells survived temporary exposure to high Na₂S concentrations, their growth was not stimulated by 1 × 10^?5 to 1.0 M Na₂S. Possible metabolic roles of reduced sulfur compounds are considered. Microcystis colonies disaggregated to unicells at ionic concentrations below 1 mM for univalent cations, 10–100 μM for the divalent cations, and 3–10 μM for Fe³⁺. Higher cation concentrations prompted cell aggregation. With > 100 mM Fe³⁺, the Microcystis capsule appeared rust-colored. Neither nonionic solutes nor anions detectably influenced aggregation. These observations suggest cation interactions with the Microcystis capsule and are discussed with regard to: a) possible siderochrome activity, cation chelation or luxury uptake of cations, b) the questionability of using cell aggregation as a criterion for identifying Microcystis in samples of unknown ionic strength, c) the utility of low ionic strength media in releasing contaminating bacteria from the capsule and in obtaining algal unicells for cloning, and d) a model for cation interactions with the capsule.     

Interspecific variation in extracellular polysaccharide content and colony formation of <Emphasis Type="Italic">Microcystis</Emphasis> spp. cultured under different light intensities and temperatures

Single cells of five different Microcystis species (M. ichthyoblabe, M. viridis, M. flos-aquae, M. wesenbergii, and M. aeruginosa) were batch-cultured at different temperatures and light intensities: (a) 25 °C and 50 μmol photons m^?2 s^?1 (control culture); (b) 25 °C and 10 μmol photons m^?2 s^?1; and (c) 15 °C and 50 μmol photons m^?2 s^?1. The extracellular polysaccharide content was significantly higher in treatments b and c than in the control treatment. All Microcystis species existed as single cells under the control treatment but formed colonies in treatments b and c. All of the colonies were irregular with indistinct margins. M. ichthyoblabe, M. viridis, M. flos-aquae, and M. wesenbergii formed colonies with similar morphologies and their cells were loosely aggregated. In contrast, M. aeruginosa formed denser colonies with no distinct holes. The colony morphologies differed from the classic morphology of M. ichthyoblabe field-grown colonies but resembled that of small colonies found in Lake Taihu (Yangtze Delta Plain, China) during early spring. This indicates that field- and laboratory-grown colonies are governed by similar formation processes. We suggest that in laboratory and field environments, M. ichthyoblabe (or M. flos-aquae) colonies are representative of small colonies formed from single Microcystis cells, whereas the morphology of older colonies evolves to resemble M. wesenbergii and M. aeruginosa colonies.     

Fate and Persistence of Particulate and Dissolved Microcystin-LA from Microcystis Blooms

Few studies have estimated fate and persistence of the hepatotoxic microcystins (MCs) in situ, making ecological and human health risk assessments challenging. We determined fate and persistence of MC congeners during 2 years of Microcystis blooms in a small, shallow, closed-basin lake in Ontario, Canada. In situ half-lives were compared to estimates obtained in vitro under controlled temperature and light. The blooms produced elevated microcystin-LA (MC-LA) (maximum ～4.2 mg L^?1) with minor concentrations of MC-LR, -RR, and -YR. Dissolved MC-LA declined more slowly and persisted longer than particulate MC-LA with in situ half-lives (total 1.5–8.5 days) shorter than in vitro (total 6.8–60.0 days). Half-lives in 2010 were two to eight times shorter compared to 2009, likely due to differences in bloom phenology and species/strain composition. In vitro, higher temperature (4°C → 25°C in dark), and irradiance (dark → 45 → 260 μE m^?2s^?1 at 25°C) accelerated particulate and dissolved MC-LA decline, respectively. MC-RR accumulated in surface sediments while MC-LA was near detection despite elevated surface water concentrations. MC-LA appears to persist longer in surface waters than the equally toxic MC-LR, requiring almost the entire recreational season (9.5 weeks) to reach guideline concentrations (20 μg L^?1).     

MEASUREMENT OF IN SITU SPECIFIC GROWTH RATES OF MICROCYSTIS (CYANOBACTERIA) FROM THE FREQUENCY OF DIVIDING CELLS1

Diel changes in the frequency of dividing cells (FDC) of three Microcystis species were investigated in a small eutrophic pond from July to October 2005. The representative species was M. aeruginosa (Kütz.) Kütz., constituting 57%–86% of the Microcystis population throughout the study period, and the remainder were M. viridis (A. Braun) Lemmerm. and M. wesenbergii (Komárek) Komárek. The FDC of M. aeruginosa and M. wesenbergii increased in the daytime and fell in the nighttime in July and August, but this regular variation was not observed in September or October. The in situ specific growth rates of Microcystis species were estimated based on the assumption that the specific growth rate can be given as an absolute value of the derivative of FDC with respect to time. The calculated values were similar among species—0.15–0.38 · d^?1 for M. aeruginosa, 0.14–0.63 · d^?1 for M. viridis, and 0.18–0.61 · d^?1 for M. wesenbergii. The specific growth rates in July and August slightly exceeded those in September and October. The analysis of the in situ specific growth rate of Microcystis indicated that recruitment of the benthic population or morphological change, rather than massive growth, was at least partly responsible for the dominance of M. aeruginosa in the study pond.     

Winter limnology: a comparison of physical,chemical and biological characteristics in two temperate lakes during ice cover

The winter dynamics of several chemical, physical, and biological variables of a shallow, polymictic lake (Opinicon) are compared to those of a deep, nearby dimictic lake (Upper Rock) during ice cover (January to early April) in 1990 and 1991. Both lakes were weakly inversely thermally stratified. Dissolved oxygen concentration was at saturation (11–15 mg l⁻¹) in the top 3 m layer, but declined to near anoxic levels near the sediments. Dissolved oxygen concentrations in the deep lake were at saturation in most of the water column and approached anoxic levels near the sediments only. Nutrient concentrations in both lakes were fairly high, and similar in both lakes during ice cover. Total phosphorus concentrations generally ranged between 10–20 μg l⁻¹, NH₄-N between 16–100 μg l⁻¹, and DSi between 0.9–1.9 mg l⁻¹; these concentrations fell within summer ranges. NO₃-N concentrations were between 51–135 μg l⁻¹ during ice cover, but occurred at trace concentrations (<0.002 μg l⁻¹) during the summer. The winter phytoplankton community of both lakes was dominated by flagellates (cryptophytes, chrysophytes) and occasionally diatoms. Dinoflagellates, Cyanobacteria and green algae were poorly represented. Cryptophytes often occurred in fairly high proportions (20–80%) throughout the water column, whereas chrysophytes were more abundant just beneath the ice. Zooplankton population densities were extremely low during ice cover (compared to maximum densities measured in spring or summer) in both lakes, and were comprised largely of copepods.