Onset Of Thermal Stratification Research Articles

Local and continental-scale controls of the onset of spring phytoplankton blooms: Conclusions from a proxy-based model.

A key phenological event in the annual cycle of many pelagic ecosystems is the onset of the spring algal bloom (OAB). Descriptions of the factors controlling the OAB in temperate to polar lakes have been limited to isolated studies of single systems and conceptual models. Here we present a validated modelling approach that, for the first time, enables a quantitative prediction of the OAB and a systematic assessment of the processes controlling its timing on a continental scale. We used a weather-driven, one-dimensional lake model to simulate the seasonal dynamics of the underwater light climate in 16 lake types characterized by the factorial combination of four lake depths with four levels of water transparency. We did so at 1962 locations across Western Europe and over 31years (1979-2009). Assuming that phytoplankton production is light-limited in winter, we identified four patterns of OAB control across lake types and climate zones. OAB timing is controlled by (i) the timing of ice-off in ice-covered clear or shallow lakes, (ii) the onset of thermal stratification in sufficiently deep and turbid lakes and (iii) the seasonal increase in incident radiation in all other lakes, except for (iv) ice-free, shallow and clear lakes in the south, where phytoplankton is not light-limited. The model predicts that OAB timing should respond to two pervasive environmental changes, global warming and browning, in opposite ways. OAB timing should be highly sensitive to warming in lakes where it is controlled by either ice-off or the onset of stratification, but resilient to warming in lakes where it is controlled by incident radiation. Conversely, OAB timing should be most sensitive to browning where it is controlled by incident radiation, but resilient to browning where it is controlled by ice-off or the onset of stratification. Available lake data are consistent with our findings.

Long‐term limnological changes in the Ecuadorian páramo: Comparing the ecological responses to climate warming of shallow waterbodies versus deep lakes

Abstract Páramos are high‐altitude ecosystems of grasslands and shrubs that sustain high levels of biodiversity and contain numerous lakes, ponds and wetlands that are a crucial source of water for millions. In the Andes, limnological data are rare from páramos and particularly so from shallow waterbodies that are prominent features of the landscape. Here, we analyse fossil diatom assemblages using dated sediment cores from three shallow lakes in the páramo of southern Ecuador and document their response to recent climate changes. The two shallowest sites were <0.5 m deep and contained nearly identical diatom assemblages, dominated by Achnanthidium minutissimum and other benthic taxa. The deepest study site, at 4 m depth, differed notably from its shallower counterparts in that the dominant taxa were tychoplanktonic Aulacoseira species. All three study sites showed only minor assemblage shifts over the past ˜200 years. This contrasts sharply with paleolimnological data from nearby deep lakes (Zmax > 17 m) that recorded abrupt changes in diatom phytoplankton coincident with the onset of higher temperatures, reduced wind speeds and the onset of thermal stratification in recent decades. In temperate and high‐latitude regions, an overriding factor influencing freshwater ecosystems is the duration and extent of ice cover, which itself is closely linked to climate variables. In contrast, there is no winter ice cover and the growing season is continuous year round in our equatorial Andean sites. Instead, rising temperatures are affecting deep lakes primarily by altering the physical structure of the water column resulting in greater periods of thermal stratification, which in turn drives changes in biota and other lake processes. However, this mechanism of change only affects deep lakes because shallow waterbodies are easily mixed by wind. Our data demonstrate the differential response of large, deep lakes in the páramo compared to the relative complacency of changes in shallow waterbodies, which is in marked contrast to similar sites affected by seasonal ice cover.

Zooplankton predators and prey: body size and stable isotope to investigate the pelagic food web in a deep lake (Lake Iseo, Northern Italy)

<p>Seasonal changes in trophic position and food sources of deep subalpine lake (Lake Iseo, Northern Italy) zooplankton taxa were investigated during the year 2011. Furthermore, it's combined carbon and nitrogen Stable Isotope Analysis (SIA) with size-specific analyses of both, the major predatory cladoceran (<em>Leptodora kindtii</em><em>,</em> Focke) and two potential preys (<em>Daphnia</em> <em>longispina</em> complex and <em>Eubosmina longicornis</em>). SIA studies have been extremely useful to track the energy flow through complex trophic network, however, if it is applied to analyze relation between two/few species may lead to misleading interpretations. In fact, integrating size-specificity allowed for understanding why <em>L. kindtii</em> nitrogen isotopic fingerprint fully overlapped with <em>Daphnia</em>, in spring. By investigating changes in <em>L. kindtii</em>'s feeding basket, we found that in spring, <em>L. kindtii </em>mainly relied upon <em>E. longicornis </em>as prey, <em>Daphnia</em> being of too large body size for being captured by <em>L. kindtii.</em> Among preys encountered directly in front by a free-swimming <em>Leptodora</em>, only those able to fit into basket opening can be captured. As basket diameter increases with animal body length, size selection of prey depends on <em>L. kindtii</em> body length. As in other deep, subalpine lakes, <em>E. longicornis </em>was less <sup>15</sup>N-enriched than <em>Daphnia</em>, most likely because of exploiting nitrogen fixing, cyanobacteria colonies, commonly detected in Lake Iseo with the onset of thermal stratification. Cyclopoid adults were at the top of zooplankton food chain and they could potentially be feeding on <em>Daphnia</em>. They, however, likely fed in a different habitat (>20 m deep water), as suggested by a rather than negligible carbon fractionation. The results overall suggest that size-specificity is crucial for addressing space and time changes in trophic links between organisms composing the two hierarchical levels within open water zooplankton community.</p><p> </p>

Altitudinal dependence of a statistically significant diel temperature cycle in Scottish lochs

Based on near-surface temperature measurements conducted at high temporal resolution in 16 Scottish lochs in 2001, a strong seasonal cycle was observed in the magnitude of the diel surface temperature range (DSTR). The influence of 5 site-specific potential predictor variables including altitude, latitude, longitude, lake surface area, and maximum lake depth on the timing of the start and end of a statistically significant DSTR was investigated. The DSTR is important because it is expected to have implications for many processes that affect biogeochemical cycling in lakes, including the rates of metabolic processes. The timing of the start of a statistically significant DSTR in spring showed a strong dependence on altitude, a much weaker dependence on latitude, but no dependence on any other predictor variable tested. The timing of the end of a statistically significant DSTR in autumn showed no dependence on any of the predictor variables tested. Because the start of a statistically significant DSTR is dependent on altitude but its end is not, its duration is also dependent on altitude. Because the start of a significant DSTR is related to the onset of thermal stratification, the results suggest that the altitude of a lake will affect the timing of the onset of thermal stratification and its duration.

Dynamics of Aphanizomenon and Microcystis (cyanobacteria) during experimental manipulation of an urban impoundment

Ford Lake in southeastern Michigan has a history of midsummer cyanobacterial blooms marked by a regular succession from heterocystous Aphanizomenon flos-aquae to nonheterocystous Microcystis aeruginosa. Nitrogen (N)-fixing Aphanizomenon has an advantage following seasonal thermal stratification because nitrate is consumed by assimilative uptake in the epilimnion and by anaerobic respiration in the anoxic hypolimnion. If nitrate levels increase, however, Microcystis succeeds Aphanizomenon. Bioassays corroborated the importance of nitrate to Microcystis success in late summer. Experimental releases of hypolimnetic water from the hydroelectric outlet dam before anoxia developed in 2006 destabilized the water column without altering lake volume and promoted a diatom bloom rather than cyanobacteria. To establish that the experimental manipulation was reproducible, we followed 2006 with a control year and then a second experimental year. In 2007, experimental manipulation was not performed, water was withdrawn from the epilimnion alone, and the lake reverted to historical dominance by first Aphanizomenon and then Microcystis. In 2008 we again commenced hypolimnetic discharge at onset of thermal stratification, and a sequence of diatom blooms resulted, demonstrating that community transformation first produced in 2006 was reproducible. The algal community again became transformed from cyanobacteria to diatoms, but overall biomass did not change. Bioassays demonstrated that Aphanizomenon became P limited during diatom dominance. Cyanobacterial blooms were postponed until after termination of the whole lake experiment and subsequent diatom decline.

Role of light and photophysiological properties on phytoplankton succession during the spring bloom in the north-western Mediterranean Sea

This study aimed to determine the role of light on the succession of the phytoplankton community during the spring bloom in the northwestern Mediterranean Sea. To this end, three successive Lagrangian experiments were carried out between March and April 2003. The three experiments correspond to distinct phases of the bloom development (pre-bloom, bloom peak and post-bloom, respectively) and therefore to different trophic conditions. Phytoplankton (sampled on a daily scale) was grouped in size-based classes (pico and nano+micro) each of them were characterised in terms of chemotaxonomic composition, primary production and photophysiological properties. The phytoplankton community evolved with time changing in both size-class dominance and specie/group dominance within each size class. The bloom peak was characterised by highly dynamic condition (i.e. vertical mixing) and by the dominance of both small (pico) and large (nano and micro) diatoms, as a result of their capacity to photoacclimate to changing light regimes (‘physiological plasticity’). Concluding, we suggest that the physiological adaptation to light is the main factor driving the succession of the phytoplankton community during the first phases of the bloom (until the onset of thermal stratification) in the western Mediterranean Sea.

Role of light and photophysiological properties on phytoplankton succession during the spring bloom in the north-western Mediterranean Sea

This study aimed to determine the role of light on the succession of the phytoplankton community during the spring bloom in the northwestern Mediterranean Sea. To this end, three successive Lagrangian experiments were carried out between March and April 2003. The three experiments correspond to distinct phases of the bloom development (pre-bloom, bloom peak and post-bloom, respectively) and therefore to different trophic conditions. Phytoplankton (sampled on a daily scale) was grouped in size-based classes (pico and nano+micro) each of them were characterised in terms of chemotaxonomic composition, primary production and photophysiological properties. The phytoplankton community evolved with time changing in both size-class dominance and specie/group dominance within each size class. The bloom peak was characterised by highly dynamic condition (i.e. vertical mixing) and by the dominance of both small (pico) and large (nano and micro) diatoms, as a result of their capacity to photoacclimate to changing light regimes (‘physiological plasticity’). Concluding, we suggest that the physiological adaptation to light is the main factor driving the succession of the phytoplankton community during the first phases of the bloom (until the onset of thermal stratification) in the western Mediterranean Sea.

Seasonal zooplankton dynamics in Lake Michigan: Disentangling impacts of resource limitation, ecosystem engineering, and predation during a critical ecosystem transition

We examined seasonal dynamics of zooplankton at an offshore station in Lake Michigan from 1994 to 2003 and 2007 to 2008. This period saw variable weather, declines in planktivorous fish abundance, the introduction and expansion of dreissenid mussels, and a slow decline in total phosphorus concentrations. After the major expansion of mussels into deep water (2007–2008), chlorophyll in spring declined sharply, Secchi depth increased markedly in all seasons, and planktivorous fish biomass declined to record-low levels. Overlaying these dramatic ecosystem-level changes, the zooplankton community exhibited complex seasonal dynamics between 1994–2003 and 2007–2008. Phenology of the zooplankton maximum was affected by onset of thermal stratification, but there was no other discernable effect due to temperature. Interannual variability in zooplankton biomass during 1994 and 2003 was strongly driven by planktivorous fish abundance, particularly age-0 and age-1 alewives. In 2007–2008, there were large decreases in Diacyclops thomasi and Daphnia mendotae possibly caused by food limitation as well as increased predation and indirect negative effects from increases in Bythotrephes longimanus abundance and in foraging efficiency associated with increased light penetration. The Bythotrephes increase was likely driven in part by decreased predation from yearling and older alewife. While there was a major decrease in epilimnetic–metalimnetic herbivorous cladocerans in 2007–2008, there was an increase in large omnivorous and predacious calanoid copepods, especially those in the hypolimnion. Thus, changes to the zooplankton community are the result of cascading, synergistic interactions, including a shift from vertebrate to invertebrate planktivory and mussel ecosystem impacts on light climate and chlorophyll.

Annual cycles and spring blooms in phytoplankton: don’t abandon Sverdrup completely

The critical-depth model for the onset of the spring phytoplankton bloom in the North Atlantic has recently been called into question by several researchers. The critical-depth model considers that the spring bloom starts when the mixed layer shoals to become shallower than a critical depth. Satellite and in situ measurements of chlorophyll are used here to show that the critical-depth model is indeed flawed. It is shown that the critical-depth model does not apply in the spring because the basic assumption of an upper layer that is well-mixed in plankton is not met. Instead, the spring bloom forms in shallow near-surface layers that deepen with the onset of thermal stratification. A stratification-onset model for the annual cycle in plankton is proposed that adheres to the conventional idea that the spring bloom represents a change from a deep- mixed regime to a shallow light-driven regime, but where the upper layers are not well mixed in plankton in spring and so the critical-depth model does not apply. Ironically, perhaps, the critical- depth model applies in the autumn and winter when plankton are well-mixed to the seasonal thermocline, so that the critical-depth model can be used to determine whether net winter production is positive or negative.

Physical Controls on Spatial Variability in Decomposition of Organic Matter in Lake Kinneret, Israel

High resolution chemical data collected during summer 2003 indicate that the lower water mass (LWM) of the thermally stratified Lake Kinneret (LK) can be subdivided into three layers: a benthic boundary layer (BBL), overlain by the hypolimnion (HYP), and on top, the lower part of the metalimnion (ME-L). After onset of thermal stratification, the BBL is the first layer that turns anoxic, followed shortly afterward by the ME-L, while the HYP remains oxic and has relatively higher pH until later in summer. Thus, during the early summer, the HYP forms an oxygen-containing layer in-between two DO-deficient layers. Somewhat later, the HYP is characterized by still having significant levels of nitrate NO3, while in both adjacent layers nitrate is already removed through denitrification. The mechanisms controlling the gradual decline of dissolved oxygen (DO) in the HYP during the summer were studied. The seasonal mean lake-wide vertical eddy diffusion coefficient in this layer, evaluated from heat flux measurements, is approximately 4 × 10−6 m2 s−1. The vertical oxygen flux due to diffusion from within the HYP toward its oxygen-deficient upper and lower boundaries accounts for most of the slow summer decline in DO in this layer. A smaller portion of this decline can be attributed to in-layer respiratory processes. The low turbidity, relatively high pH, and slow accumulation rate of NH4 in the HYP support the notion that the slower mineralization processes occurring in this layer result from relatively low ambient concentrations of biodegradable organic matter, most probably due to the short residence time of the particles settling through this layer.

Small tropical reservoirs and fish cage culture: a pilot study conducted in Machakos district, Kenya

The findings of this study provide useful insights into the key aspects of fish cage farming to be considered during setting and overall management of multi-purpose water resources so as to maximise their aquacultural potential. The study was conducted in Machakos District, Kenya, to characterise limnochemistry and algal composition of three small man-made reservoirs Ngeki, Ngei and Kavovi, used for cage culture of Nile tilapia (Oreochromis niloticus). In Ngeki, the open water was compared with areas in between the cages and significant differences in dissolved oxygen were found. For N, P and chlorophyll-a, no differences were noticed. In all the reservoirs, temporal variations were noticed for all parameters except NO3N and NH4N. Peak concentrations of nutrients occurred during the rainy season with lower values in the dry season (below detectable limits for SRP). All reservoirs were classified as hyper-eutrophic with a strong positive correlation between chlorophyll-a and TP. Algal abundances changed with season. Chlorophyta dominated at the start of the rainy season, while cyanobacteria prevailed at the onset of thermal stratification. Diatoms prevailed in the dry season but were notably absent in the rainy season. It was concluded that all the reservoirs are suitable for cage culture of Nile tilapia, but this should be undertaken with caution considering critical periods when anoxic conditions may occur.

Thermal stratification, mixing, and heat budgets of Florida lakes

We measured temperature and oxygen profiles at approximately monthly intervals over a period of 15 months in four relatively deep (z max = 17.5―24 m) Florida (USA) lakes. Johnson Pond and Lake Sheelar lie in north Florida. Lakes Tulane and Verona are in south-central Florida. All four lakes were strongly stratified thermally in summer and weakly stratified or isothermal during winter. Sheelar displayed winter isothermy at ∼17°C and Johnson Pond at ∼13 °C. The southern lakes were isothermal at ∼19 °C. Surface water temperatures exceeded 30 °C in all lakes in summer. Annual heat budgets for each lake were calculated twice, using heat loss from summer to winter and heat gain from winter to summer in 2006 and 2007. Three lakes showed similar annual heat budgets despite differences in latitude and size: Sheelar (9069 and 9955 cal cm ―2 ), Tulane (8459 and 9319 cal cm ―2 ), and Verona (7948 and 8444 cal cm ―2 ). Johnson Pond displayed lower values of 5105 and 6049 cal cm ―2 . Johnson Pond had lower isothermic winter temperatures. It also had smaller annual heat budgets because heat is not transferred effectively to great depth in the lake during the period when thermal stratification is established. This is a consequence of wind protection by surrounding vegetation, short fetch, and high amounts of dissolved and suspended material in the water. All four study basins have annual heat budgets that are low compared with temperate lakes of similar size and depth, but similar to values for many tropical basins. During winter circulation, oxygen penetrates to great depths in Lakes Sheelar, Tulane and Verona. The entire water column was oxygenated in Lake Sheelar during winter isothermy. Lakes Tulane and Verona, however, failed to oxygenate completely during winter mixing. Furthermore, oxygen was quickly depleted in deep waters of these relatively unproductive lakes after the onset of thermal stratification in spring. Johnson Pond was nearly completely deoxygenated during winter mixing. This was probably a consequence of dilution of oxygen in surface waters by deeper anoxic water and consumption of O 2 by reduced ions that accumulate in the hypolimnion.

The seasonal evolution of high vertical‐mode internal waves in a deep reservoir

The causal mechanism and seasonal evolution of the internal wave field in a deep, warm, monomictic reservoir are examined through the analysis of field observations and numerical techniques. The study period extends from the onset of thermal stratification in the spring until midsummer in 2005. During this time, wind forcing was periodic, with a period of 24 h (typical of land‐sea breezes), and the thermal structure in the lake was characterized by the presence of a shallow surface layer overlying a thick metalimnion, typical of small to medium sized reservoirs with deep outtakes. Basin‐scale internal seiches of high vertical mode (ranging from mode V3 to V5) were observed in the metalimnion. The structure of the dominant modes of oscillation changed as stratification evolved on seasonal timescales, but in all cases, their periods were close to that of the local wind forcing (i.e., 24 h), suggesting a resonant response. Nonresonant oscillatory modes of type V1 and V2 became dominant after large frontal events, which disrupted the diurnal periodicity of the wind forcing.

Trophic interactions promote dominance by cyanobacteria (Anabaena spp.) in the pelagic zone of lower Karori reservoir, Wellington, New Zealand

Cyanobacterial blooms have occurred in the Lower Karori Reservoir (Wellington, New Zealand) since 2000, resulting in reduced water quality and potential health risks. This study examined the factors that contribute to the cyanobacterial blooms in the reservoir (October 2003 to July 2004). In particular, it examined how thermal stratification influences the chemical dynamics in the reservoir, and the potential cascading effect of zooplanktivorous perch (Perca fluviatilis) in promoting high phytoplankton biomass. Thermal stratification, coupled with increased water column stability occurred from October to March. Oxygen levels were reduced in the hypolimnion during stratification. Nutrient concentrations declined during stratification and became elevated after the collapse of the bloom in February 2004. Concentrations of Anabaena lemmermannii increased with the onset of thermal stratification. Zooplankton abundance peaked during stratification and consisted mainly of rotifers. The perch caught during this study were typically small‐sized individuals, which had consumed mostly large‐sized zooplankton. The development of cyanobacterial blooms in the Lower Karori Reservoir is probably owing to a combination of factors including low nitrogen concentrations that favour nitrogen‐fixing cyanobacteria and the ability of cyanobacteria to adjust their buoyancy and maintain their position in the water column during thermal stratification. Predation by perch probably maintains large zooplankton species at low densities and consequently reduces grazing pressure on phytoplankton populations.

The seasonal cycle of the epipelagic mesozooplankton in the northern Gulf of Aqaba (Red Sea)

The abundance and composition of the mesozooplankton were studied in the upper 100 m of the northern Gulf of Aqaba over a period of 22 months between February 2002 and December 2003. The samples were collected monthly with a Nansen net of 200 μm mesh size. The abundance of total mesozooplankton ranged between 245 and 3065 ind. m − 3 . Highest densities in 2002 were recorded in spring (March/April) and in autumn (October/November), while several peaks occurred in 2003 with maximum abundance in early summer (June/July). Copepods dominated the mesozooplankton community during the whole sampling period (mean 79%). Molluscs ranked secondly (8%), followed by appendicularians (2.5%) and chaetognaths (2.4%). All other taxa contributed only a small fraction of the total mesozooplankton population (0.3%). Within the copepods, calanoids were predominant throughout the sampling period (41–83%. mean 61%). Cyclopoid copepods accounted for 17 and 59% (mean 37%) while harpacticoids occurred only in very low numbers (less than 1%). Within the calanoid copepods, smaller sized taxa were abundant, belonging to the families of Clausocalanidae, Paracalanidae, Calocalanidae, Acartiidae and Mecynoceridae. Together they accounted for 84 to 98% of the calanoids. A performed cluster analysis revealed significant difference between the samples from January to May and those from June to December. Accordingly, the abundant calanoid copepod taxa could be divided into two groups: firstly, taxa with high abundance during the transition from winter–spring transition (onset of thermal stratification) and secondly, taxa with high densities during the disintegration of the stratification in autumn. The taxa of the second group showed a positive significant correlation to the annual cycle of temperature, indicating a strong relationship with seasonal changes in the hydrographic features of the Gulf of Aqaba.

Twenty years of spatially coherent deepwater warming in lakes across Europe related to the North Atlantic Oscillation

Twenty to fifty years of annual mean deepwater (hypolimnetic) temperature data from twelve deep lakes spaced across Europe (2°95'W to 14°0'E, 46°27' to 59°00'N) show a high degree of coherence among lakes, particularly within geographic regions. Hypolimnetic temperatures vary between years but increased consistently in all lakes by about 0.1–0.2°C per decade. The observed increase was related to the weather generated by largescale climatic processes over the Atlantic. To be effective, the climatic signal from the North Atlantic Oscillation (NAO) must affect deep lakes in spring before the onset of thermal stratification. The most consistent predictor of hypolimnetic temperature is the mean NAO index for January–May (NAOJ–M), which explains 22–63% of the interannual variation in deepwater temperature in 10 of the 12 lakes. The two exceptions are remote, less wind‐exposed alpine valley lakes. In four of the deepest lakes, the climate signal fades with depth. The projected hypolimnetic temperature increase of approximately 1°C in 100 yr, obtained using a conservative approach, seems small. Effects on mixing conditions, thermal stability, or the replenishment of oxygen to deep waters result in accumulation of nutrients, which in turn will affect the trophic status and the food web.

Spawning events in small and large populations of the green sea urchin Strongylocentrotus droebachiensis as recorded using fertilization assays

During the winter and spring of 2002 and 2003, we used time-integrated fertilization assays to monitor sperm availability in three populations of the green sea urchin Strongylocentrotus droebachiensis in Maine: a naturally occurring population of >40,000 urchins and two smaller groups (<1,000) of transplanted urchins isolated from other aggregations. Episodes of sperm release coincided in two populations 10 km apart, suggesting that urchins were responding to a widespread environmental signal. We observed significant lunar periodicity in sperm release events for both of these populations. However, extensive spawning as shown by fertilization rates near 100% and a dramatic drop in gonad mass only occurred in the large natural population around the onset of thermal stratification and the spring phytoplankton bloom. By contrast, in the two small populations we observed low fertilization rates and little or no change in gonad mass. We speculate that a subset of males in these populations responded to a common external spawning signal, but that mass spawning is more likely to occur in large, dense populations where sperm concentrations reach high enough levels to trigger spawning in less responsive urchins.

Thermal Characteristics and Energy Balance of Various-Size Canadian Shield Lakes in the Mackenzie River Basin

This study addresses the thermal and energy budget characteristics of four different-size Canadian Shield lakes in the Mackenzie River basin during the ice-free season of 2000. The objectives are to characterize and compare the surface temperature and thermal structures, and to quantify the magnitudes and flux rates of the energy balance components of each lake. This study highlights the variability in thermal and energy balance characteristics arising from differences in mean lake depth and surface area. The lakes exhibit similar temporal patterns for air temperature, net radiation, wind speed, and wind direction. Net radiation and wind speed are highest over the largest lake, Great Slave Lake, due to colder surface temperatures and lengthy across-lake wind fetch, respectively. During the warming phase of the summer, surface temperature is inversely related to mean depth; however, during the cooling phase this relationship reverses. The shallowest of the four lakes remains isothermal during the entire ice-free period, while the three larger and deeper lakes are all dimictic. A lag in the onset of thermal stratification in the dimictic lakes is positively correlated with mean depth and surface area. Large evaporative water losses correspond to periods of low net radiation and cold dry air over Great Slave Lake. However, over the smaller, shallower lakes periods of high evaporation occur on days with high net radiation and warm, dry air. The capacity of larger lakes to store more heat results in longer ice-free periods and higher evaporation. Maximum heat content increases and occurs later for lakes of greater depth. Maximum evaporative rates occur later and cumulative evaporation is highest for lakes of greater depth and area. The ratios of total open water evaporation for the four lakes in order of size (smallest 5 1.0) are 1.0 : 1.2 : 1.3 : 1.4. Evaporation magnitudes are discussed in the context of other temperate and high-latitude lake studies.

Dinoflagellate bloom development and collapse in Lake Kinneret: a sediment trap study

Warm monomictic Lake Kinneret, Israel, is characterized by a winter–spring water bloom of the large (∼50 μm diameter) dinoflagellate Peridinium gatunense Nygaard. Usually the P. gatunense bloom declines in May–June and a less prominent bloom of smaller dinoflagellates (mostly Peridiniopsis spp. of ∼20–30 μm diameter) develops. Water column abundances and sedimentation losses to those dinoflagellates were followed throughout 1994 and 1995. The objective was to quantify the variables that describe population dynamics, that in turn will shed more light on the seasonal patterns of bloom dynamics. Sedimentation losses were measured by means of sediment traps with and without a preservative (formaldehyde) that were exposed for 24 h once every 2–3 weeks. Annual sedimentation losses of Peridinium (hypolimnetic trap catches) were 209 g wet wt m−2 year−1 in 1994 and 187 g wet wt m−2 year−1 in 1995, which constituted 16 and 23% of Peridinium production in those years, respectively. This study revealed that increased death rates preceded a mass sedimentation flux of Peridinium and caused the decline of the bloom in Lake Kinneret. Annual sedimentation losses of Peridiniopsis were 55 g wet wt m−2 year−1 in 1994 and 34 g wet wt m−2 year−1 in 1995. In contrast to live Peridinium cells, Peridiniopsis cells continued to swim to the lower trap after the onset of thermal stratification, possibly taking advantage of the higher nutrient concentrations below the thermocline, at a time when the lake is already stratified and the epilimnion is nutrient depleted. This could be an important factor allowing Peridiniopsis spp. to peak after the decline of Peridinium.

Perennial domination of phytoplankton by Botryococcus and Peridinium in a discontinuously polymictic reservoir (tropical Australia)

The effect of holomixis and wet season inflow on the phytoplankton composition of Darwin River Reservoir (DRR) is examined. Both phenomena constitute potential disturbances to the pelagic environment of this shallow (average depth 6 m) tropical reservoir and its phytoplankton community. Over the two year study period, the phytoplankton concentration of DRR remained much the same (0.48-2.4 mm(3)/L). At least half the biovolume comprised two species, Botryococcus braunii and Peridinium gutwinskii. These two taxa, together with other Peridinium species and desmids (predominantly Staurastrum and Cosmarium) constituted 68-98% of the reservoir's total phytoplankton biovolume. Potential disturbance by wet season inflow and 13 holomictic events had minimal affect on the domination of the Botryococcus-Peridinium-Desmidaceae assemblage, owing mainly to their short duration (4-5 days) and minor influence on light intensities in the mixed layer. Some holomictic events, however, favoured the growth of the diatom Aulacoseira granulata which reached a maximum of 17 % of the reservoir's total phytoplankton biovolume. Fast growing, small r-selected phytoplankton, which are favoured by nutrient rich waters, constituted a minor component (<1 % total biovolume) of the reservoir's phytoplankton, probably owing to the phosphorus impoverishment of the mixed layer. B. braunii colonies and P. gutwinskii have many characteristics of K-selected (slow growing) S-species, which mark the later stages of autogenic succession. There was no evidence of phytoplankton autogenic succession with the onset of thermal stratification in DRR. The perennial domination of the reservoir's phytoplankton by a single assemblage, and notably by two species, suggests the reservoir's phytoplankton community is in dynamic equilibrium (or steady-state) with the environment, supporting the contention that climax communities are self-sustaining.