Late Stratification Research Articles

The wind‐driven distribution of nearshore zooplankton in a stratified lake varies with their body size

Abstract Wind‐driven forces are expected to concentrate zooplankton along downwind shores in lakes and provide food subsidy to nearshore food webs, but the bathymetric complexity of nearshore areas could result in high spatial variability. Here we test: (1) whether zooplankton accumulate downwind on windy days, and whether the magnitude of accumulation varies with zooplankton body size and with nearshore bathymetric slope; and (2) whether nearshore zooplankton are more spatially variable than offshore zooplankton, and whether their spatial variability is related to wind conditions and to zooplankton body size. This study focuses on zooplankton distribution at the whole‐lake basin and at intermediate (10 m–1 km) spatial scales. Zooplankton were sampled repeatedly along four c. 1 km transects at each of the upwind and downwind ends of a 22.1‐km2 stratified lake basin. Two transects were oriented perpendicular to shore and two were parallel, one close to shore and the other further offshore. Zooplankton were sampled along each transect using an optical plankton counter morning and afternoon over 3 consecutive days, under a range of wind conditions, during early stratification (early June), mid‐stratification (mid‐July) and late stratification (late September). A 3D‐hydrodynamic model was used to help interpret the results. Small‐bodied zooplankton accumulated downwind during windy periods, reaching 8%–34% higher biomass than the basin‐wide average. This downwind accumulation of biomass was best explained by a partial (metalimnetic) upwelling displacing the epilimnion to the downwind end of the lake basin. Active upward movement of zooplankton is required to explain near‐surface biomass accumulation in this displaced water mass. Large‐bodied zooplankton accumulated in nearshore areas, regardless of wind conditions. Higher nearshore accumulation of large zooplankton was observed along transects following a shallow (1.9%) bathymetric slope but not along a steep (6%) slope. Large‐bodied zooplankton are more patchily distributed than small‐bodied zooplankton, both nearshore and offshore. The spatial variability of large zooplankton declined with increasing wind speed, but only at upwind sites. The patchy distribution of large‐bodied zooplankton is driven, or at least strongly influenced, by biological processes. The spatial distribution and patchiness of zooplankton varies with wind speed, zooplankton body size and nearshore bathymetry. Planktonic organisms are not passive drifters, and their patchiness and influx to certain parts of the lake during windy periods are both expected to affect the efficiency of trophic transfers in lake food webs.

Spirostomum teres: A Long Term Study of an Anoxic-Hypolimnion Population Feeding upon Photosynthesizing Microorganisms

The pelagic / anoxic hypolimnion population of Spirostomum teres was investigated as a part of the long-term ciliates’ monitoring (2003–2016) in an oligo- to mesotrophic monomictic hyposaline crater lake Alchichica (Puebla / Veracruz, Mexico), including an analysis of picoplankton (both heterotrophic, HPP and autotrophic, APP) and inorganic compounds of nitrogen (ammonium, nitrite, nitrate), phosphorus (dissolved reactive phosphorus, DRP) and silicon. Additionally, detailed studies of the ciliate vertical distribution and feeding activity measured upon fluorescently labelled APP (picocyanobacteria) were carried out. The results were compared with those from a neighbour freshwater crater lake La Preciosa and with a meromictic karstic lake La Cruz (Cuenca, Spain). The ciliate vertical distribution within the water column was very well defined: During the first decade, the benthic population was frequently found throughout a developing stratification of the lake. The established stratification of the lake turned the conditions favourable for the formation of an oxycline / hypolimnion population, typically, several meters below the deep chlorophyll maximum (formed basically by diatoms); the population preferred the layers without detectable dissolved oxygen. However, an observed gradient of light (PAR) could support both oxygenic and anoxygenic photosynthesis. Late stratification after deepening of the thermocline reduced the layers with S. teres population to a minimum apparently due to the drastic change in physicochemical conditions within a metalimnion, coupled with an oxycline, and limited to 1 to 2 meters; microstratification was found. Last years, the very bottom population disappeared or it was reduced and the late stratification S. teres peaks were smaller or did not appeared. Generally, S. teres oxycline / anoxic hypolimnion population was observed from June through November. Optimum picoplankton numbers in conditions that supported the ciliate growth were found: The ciliate was peaking at APP of 0.6 to 1 × 105 cells mL–1; the optimum of HPP was observed round 1.4 × 106 cells mL–1. S. teres was efficiently feeding upon picocyanobacteria in numbers of 105 cells mL–1 reaching the clearance rate of 2000 nL cell–1h–1, which represented in average 130 to 210 cells cell–1h–1 ingested. Feeding upon purple sulphur bacteria was observed but only during the end of the lake stable stratification when the ciliate population was already dropping. On the other hand, the volume specific clearance of S. teres upon picocyanobacteria (103 h–1) did not support the hypothesis that they could serve as a sole prey. Feeding upon eukaryote phytoplankton (chlorophytes Monoraphidium minutum, diatoms Cyclotella choc tawhatcheeana) could be of higher importance that previously supposed. Additionally, a use of ingested and retained photosynthetic prokaryotes is hypothesized.

Winds and the distribution of nearshore phytoplankton in a stratified lake

The distribution of phytoplankton in lakes is notoriously patchy and dynamic, but wind-driven currents and algal buoyancy/motility are thought to determine where algae accumulate. In this study, nearshore phytoplankton were sampled from different parts of a lake basin twice a day for 4–5 consecutive days, in the spring and in late summer, to test whether short-term changes in phytoplankton biomass and community composition can be predicted from wind-driven currents. On windy days, phytoplankton biomass was higher at downwind than at upwind nearshore sites, and the magnitude of this difference increased linearly with increasing wind speed. However, contrary to the generally assumed downwind phytoplankton aggregations, these differences were mostly due to upwelling activity and the dilution of phytoplankton at upwind nearshore sites. The distribution of individual taxa was also related to wind speed, but only during late stratification (except for cryptophytes), and these relationships were consistent with the buoyancy and motility of each group. On windy days, large diatoms and cyanobacteria concentrated upwind, neutrally buoyant taxa (green algae, small diatoms) were homogeneously distributed, and motile taxa (cryptophytes, chrysophytes, dinoflagellates) concentrated downwind. Predictable differences in the biomass and composition of phytoplankton communities could affect the efficiency of trophic transfers in nearshore areas.

Vertical and seasonal distribution of picoplankton and functional nitrogen genes in a high‐altitude warm‐monomictic tropical lake

Abstract The nitrogen (N) cycle is highly dependent on microbial processes. Distribution of these communities is one of the most important factors in the variation of the N cycling in warm‐monomictic lakes. However, the effects of the alternation of water stratification and mixing periods on the ecology of microbial communities involved in these processes are rarely studied in this type of aquatic ecosystem. We explored vertical and seasonal patterns of picoplankton and the genetic potential for ammonium oxidation (amoA gene for bacteria and archaea), denitrification (nirS and nirK), anammox (hzsA), and DNRA (nrfA) and their relationships with the main limnological variables in Lake Alchichica (Central Mexican Plateau) to provide insight into the distribution and importance of these planktonic communities in warm‐monomictic tropical lakes. Ten depths were sampled during late stratification (November 2015) and mixing (February 2016) periods, covering the epilimnion, metalimnion (oxycline), and hypolimnion layers in the first case. We showed that temperature and oxygen stratification shaped the distribution of picoplankton and functional N genes in this lake. These communities also varied in relation to nutrient availability and underwent temporal changes throughout the water column. The amoA genes, along with autotrophic picoplankton, were more abundant during the stratification, indicating that nitrification could be potentially more important during this period, mainly at the oxycline layer. Denitrifying genes showed strong variations during the stratification period, with highest gene copy numbers at the oxycline and hypolimnion layers. Anoxic conditions were characterised by a relative increase in the abundance of the nrfA gene with depth, which was positively correlated with NH4+ concentration. On the other hand, the hzsA gene was not detected in any sample. Our findings highlight the importance of thermal stratification as one of the main factors influencing the genetic potential for N transformations within the water column in warm‐monomictic tropical lakes.

Inter-annual ciliate distribution variation within the late stratification oxycline in a monomictic lake, Lake Alchichica (Mexico)

<p>noxia, and ciliates might play a very important role in the plankton community budget there. We analysed changes in the composition and biomass of the ciliate assemblage and other microbial loop components throughout the oxycline just at the end of stratification in a warm-monomictic lake, Lake Alchichica, Mexico (four samplings: 2006-2008, 2010); the results were compared with those obtained from another lake from the region, La Preciosa, sampled in 2010. Bacteria, autotrophic picoplankton (APP) and flagellates were analysed using epifluorescence microscopy. Ciliates were evaluated either in DAPI stained samples (looking for pigmented organelles and/or ingested phototrophs) or in quantitative protargol stain (QPS) permanent preparations, where they were identified at the genus or species level. The end of the stratification period in Lake Alchichica was characterized by almost uniform heterotrophic picoplankton (HPP) numbers (10<sup>6</sup> cells mL<sup>-1</sup>) throughout the water column. Meanwhile, APP showed epilimnetic and/or metalimnetic maxima of 10<sup>5</sup> cells mL<sup>-1</sup> followed by an order of magnitude drop in the hypolimnion. A very important peak (10<sup>5</sup> cells mL<sup>-1</sup>) of the autotrophic or mixotrophic flagellate <em>Pyramimonas</em> sp. was observed repeatedly above and within the oxycline of Lake Alchichica. Ciliate biomass maxima were found around the oxycline and in the above-bottom layer. The top of the oxycline was dominated by <em>Euplotes</em> spp. and <em>Spirostomum teres</em> fine- to coarse-filter feeders (feeding upon APP, nanodiatoms and algae). Raptorial haptorids (in particular, <em>Phialina </em>sp.) were the second most important group, generally occupying the layer below euplotids, followed by <em>Holophrya</em> and <em>Prorodon</em> facultative anaerobic prostomes. Sometimes, strictly anaerobic <em>Caenomorpha</em> sp. was found to be important in the anoxic hypolimnion. Minute picoplankton feeding species (both APP and heterotrophic bacteria feeders) were important throughout the water column: in the epilimnion, vorticellids (2006-2008) or scuticociliates (2010) dominated. Typically, the scuticociliate maximum was located in the oxycline and/or above the bottom. Some microaerophilic species were isolated; thus, their identification could be carried out. However, the apparent polymorphic ciliate life cycles were not described completely, and the species composition was only estimated: two dominant species (SC 1 – <em>Cristigera</em>-like and SC 2 – <em>Cyclidim</em>-like) covered nearly the total scuticociliate biomass. Strictly anaerobic scuticociliates were not isolated but observed in the deepest layers of the lake (bacteria symbiotic <em>Isocyclidium globosum</em> and <em>Cristigera </em>sp.). Significant statistical relation within the ciliate distribution and environmental variables was not confirmed due to unique species composition in the respective years. However, general trends in the distribution of ciliates on a species level were observed. Scuticociliates, including two important tentatively identified species, did not present unambiguous ecological position, and the study of their live cycle should be the next step in investigations.</p>

Trace elements influenced by environmental changes in Lake Biwa: (II) Chemical variations in the hypolimnion over the last half-century

The history of the deep north basin of Lake Biwa extends over 430,000 years. Although it has probably been oxic and oligotrophic since its formation, human impacts have been changing lake conditions. In this paper, we discuss long-term changes in the chemistry of bottom water by compiling literature and through our own data over the last half-century. Long-term records show an increase in temperature, decrease in dissolved oxygen (DO), and increase in nutrients in bottom water. The stoichiometry among oxygen and nutrients indicates that changes are basically consistent with aerobic decomposition of organic matter. These changes are most likely the result of global warming and local eutrophication. Of particular note, yearly minimum DO concentrations <50 µmol kg−1 have started to occur frequently at ~90 m depth since 1999. Manganese (Mn) concentrations in bottom water are at their minimum during the turnover period and at a maximum during the late stratification period each year. Yearly minimum Mn concentration has been within a narrow range over the last 30 years (0.25 ± 0.07 µmol kg−1, n = 12). However, abnormally high Mn concentrations (up to 9.3 µmol kg−1) were observed in 2007, caused by reductive release of a substantial amount of Mn from suboxic sediments and subsequent oxidation in bottom water. The concentration of arsenic (As) has gradually increased over the last 20 years in a similar manner, with a homologous element of phosphorus (P), resulting in an observed range of 17–29 nmol kg−1 in 2010. The accumulation rate was ~0.8 nmol kg−1 year−1 for As and ~6 nmol kg−1 year−1 for P.

Wind‐driven physical processes and sediment characteristics affect the distribution and nutrient limitation of nearshore phytoplankton in a stratified low‐productivity lake

Lay AbstractWind‐driven physical processes are expected to affect the spatial distribution and composition of algae in lakes and reservoirs, and to determine their access to nearshore nutrients. We used nutrient addition bioassays to detect changes in the nutrient status of phytoplankton, which indicate changes in nutrient availability in the water. We examined these effects at offshore and nearshore sites and at different times of the year, under different stratification conditions (prestratification, early and late stratification) and with different phytoplankton communities. Phytoplankton accumulated downwind, but their growth rate was usually higher at upwind than downwind sites. This suggests that the quantity and quality of algal food sources for higher trophic levels may vary in predictable but opposite ways. Wind‐driven surface waves and upwelling activity were associated with changes in phytoplankton nutrient limitation in nearshore areas, but these differences were site specific. Our results suggest that wind‐driven physical processes and sediment characteristics are both important in determining internal nutrient loading and phytoplankton nutrient limitation in nearshore areas. On windy days, nutrient limitation of offshore phytoplankton at the lake surface was always related to the conditions found upwind, suggesting rapid exchanges between nearshore and offshore areas. Wind‐driven physical processes affect the distribution and nutrient limitation of phytoplankton in lakes, and are likely to influence the efficiency of energy transfers through planktonic food webs. These wind‐driven processes should be included more specifically into food web models.

A story of famine in the pelagic realm: temporal and spatial patterns of food limitation in rotifers from an oligotrophic tropical lake

Food limitation is one of the most important extrinsic factors influencing population dynamics in aquatic ecosystems, but the spatial patterns of resource limitation and their influence on zooplankton species have not been investigated. We tested the hypothesis that the occurrence and intensity of food limitation may vary both spatially and temporally, and that the irregular population dynamics of the rotifers Brachionus group plicatilis and Hexarthra jenkinae in a saline, warm-monomictic, oligotrophic lake (Alchichica, Central Mexico) can be attributed to resource limitation. We estimated plankton abundance and composition and performed 19 laboratory experiments to assess the intensity of food limitation (Dr) at three selected depths in the water column. Most of the phytoplankton biomass consisted of algae inedible to rotifers, and good-quality food ( phytoplankton � 20 mm þ small heterotrophic flagellates) was frequently below the threshold food concentration for Brachionus species. The vertical distribution and abundance of rotifers were correlated with the availability of food resources. Food limitation was observed during mixing and late stratification; however, the intensity of limitation was rather similar throughout the water column owing to the overall scarcity of resources in the lake. Hexarthra seldom appeared. We conclude that the irregular pattern of rotifer abundance we observed is a consequence of the interplay between the harsh conditions of scarce food availability, resulting in intense resource competition and opportunities for rapid exploitation of opportunities when food outbursts occur.

Preferences of phytoplankton groups for waters of different trophic status in the northwestern Mediterranean Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 407:27-42 (2010) - DOI: https://doi.org/10.3354/meps08559 Preferences of phytoplankton groups for waters of different trophic status in the northwestern Mediterranean Sea Mikel Latasa1,4,*, Renate Scharek2, Montserrat Vidal3, Gemma Vila-Reixach3, Andrés Gutiérrez-Rodríguez1, Mikhail Emelianov1, Josep M. Gasol1 1Institut de Ciències del Mar (CSIC), Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain 2Centro Oceanográfico de Gijón (IEO), Avda. Príncipe de Asturias 70 bis, 33213 Gijón, Spain 3Departament d’Ecologia, University of Barcelona (UB), Diagonal 645, 08028 Barcelona, Spain 4Present address: Centro Oceanográfico de Gijón (IEO), Avda. Príncipe de Asturias 70 bis, 33213 Gijón, Spain *Email: latasa@gi.ieo.es ABSTRACT: We examined the preferences of phytoplankton groups for waters of different trophic status by comparing the distribution of 8 main phytoplankton groups during the spring bloom, post-bloom, and late stratification periods in the northwestern Mediterranean. Pigment chemotaxonomy (using the CHEMTAX computer program) was applied to estimate the contribution of Prymnesiophyceae, Pelagophyceae, Synechococcus spp., Prochlorococcus spp., Prasinophyceae, Cryptophyceae, Dinophyceae, and Bacillariophyceae to the chlorophyll a (chl a) stock. Particulate organic nitrogen (PON) concentration was used as an indicator of trophic status. PON at the surface was 1.7 ± 1.4, 0.57 ± 0.02, and 0.37 ± 0.04 µmol l–1 in the bloom, post-bloom, and stratification periods, respectively. During the bloom period, there was a weak stratification and a large chl a biomass. Bacillariophyceae dominated during the bloom period, with a substantial contribution of Prasinophyceae. Prymnesiophyceae and Synechococcus spp. dominated during post-bloom and stratification periods, and Prochlorococcus spp. was a major contributor to biomass in the deep chlorophyll maximum (DCM) during the stratification period. Vertical segregation was also evident for Pelagophyceae, Prymnesiophyceae, and Cryptophyceae, which preferred the DCM to surface layers in non-bloom conditions. The relative distribution of each group combined with PON concentrations in these 3 periods allowed us to calculate a group-specific trophic preference index, which showed its highest values (more eutrophic) for Bacillariophyceae, Prasinophyceae, Cryptophyceae, and Dinophyceae; medium values (mesotrophic) for Prymnesiophyceae, Pelagophyceae, and Synechococcus spp.; and a very low value (oligotrophic) for Prochlorococcus spp. The pigment-group diversity (Shannon index) and evenness were lower during the bloom period. KEY WORDS: Spring bloom · Post-bloom · Stratification · Eutrophic conditions · Oligotrophic conditions Full text in pdf format PreviousNextCite this article as: Latasa M, Scharek R, Vidal M, Vila-Reixach G, Gutiérrez-Rodríguez A, Emelianov M, Gasol JM (2010) Preferences of phytoplankton groups for waters of different trophic status in the northwestern Mediterranean Sea. Mar Ecol Prog Ser 407:27-42. https://doi.org/10.3354/meps08559 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 407. Online publication date: May 20, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Long term picoplankton dynamics in a warm-monomictic, tropical high altitude lake

Long term analyses of the microbial loop, centred on the picoplankton dynamics, were carried out over a five-year (1998 to 2002) period in Lake Alchichica (Puebla, Mexico), a high altitude tropical athalassohaline lake. The hydrodynamics of the lake followed a warm-monomictic pattern with mixing at a minimum temperature during the early dry season while the stratification was pronounced in the late dry season and throughout the rainy season; anoxic conditions in the hypolimnion lasted <9 months. The annual mean concentrations of chlorophyll-a were below 4 μg L-1 in 1998, 1999 and 2001, however, 6.1 and 5.2 μg L-1 in 2000 and 2002, respectively. Total picoplankton, TPP, displayed a temporal pattern that followed the mixing-stratification cycle. The highest TPP values (the whole water column ≥5×106 cells mL-1) were found during mixing and early stratification (January-March). The minimum numbers were present during late stratification (October-November). The maximum TPP numbers were observed within the layer 0-20 m, which corresponded to the epilimnion during the stratification period. Neither the thermocline nor the deep chlorophyll maximum showed an elevated TPP concentration. In the hypolimnion, TPP numbers were low (frequently <1×106 cells mL-1) apparently as a result of the long period of anoxia. Notwithstanding autotrophic picoplankton (APP) contributed even ≥30% of TPP (2001 to 2002); no significant correlation was found between TPP and chlorophyll-a.

Seasonal variation of phytoplankton nutrient limitation in Lake Erie

Phosphorus (P) has been identified as the primary nutrient limiting phytoplankton bio- mass in the Laurentian Great Lakes, and thus phytoplankton biomass varies as a function of P load- ing. While management efforts have reduced point sources of P, a recent rise in non-point source loadings and the introduction of dreissenid mussels to the Great Lakes are factors suspected to have direct and indirect impacts on nutrient cycling. We re-evaluated nutrient limitation of phytoplankton in the central basin of Lake Erie over spatial (i.e. 3 offshore stations) and temporal (i.e. monthly from June to October) scales. The nutrient limitation of the phytoplankton was evaluated using biomass response measurements to nutrient enrichment bioassays in a complete factorial of P, nitrogen (N), and silicon (Si). Nutrient additions yielded classic growth curves after 7 to 12 d of incubation. Treat- ments with added P had higher final biomass yields than treatments without P at all stations and dur- ing all months. During spring overturn, P+Si yielded higher phytoplankton biomass than did P addi- tions without Si at 2 of the 3 stations. During the late stratification to autumn overturn (August to October), P+N promoted higher phytoplankton biomass than did P additions without N; this was true at all stations. A quantitative assessment of the bioassays indicated that 63 to 94% of the biomass yield during spring overturn could be attributed to Si, while 48 to 68% of the biomass yield during late stratification and autumn overturn could be attributed to N. Water chemistry data collected in 2002 and 2003 predicted similar seasonal trends in nutrient limitation. These results suggest that sea- sonal variation in phytoplankton nutrient limitation is a probable factor in predicting changes in phytoplankton biomass and taxonomic composition in the central basin of Lake Erie.

Impact of a sea breeze on the boundary-layer dynamics and the atmospheric stratification in a coastal area of the North Sea

In-situ sodar and lidar measurements were coupled with numerical simulations for studying a sea-breeze event in a flat coastal area of the North Sea. The study's aims included the recognition of the dynamics of a sea-breeze structure, and its effects on the lower troposphere stratification and the three-dimensional (3D) pollutant distribution. A sea breeze was observed with ground-based remote sensing instruments and analysed by means of numerical simulations using the 3D non-hydrostatic atmospheric model Meso-NH. The vertical structure of the lower troposphere was experimentally determined from the lidar and sodar measurements, while numerical simulations focused on the propagation of the sea breeze inland. The sea-breeze front, the headwind, the thermal internal boundary layer, the gravity current and the sea-breeze circulation were observed and analysed. The development of a late stratification was also observed by the lidar and simulated by the model, suggesting the formation of a stable multilayered structure. The transport of passive tracers inside the sea breeze and their redistribution above the gravity current was simulated too. Numerical modelling showed that local pollutants may travel backward to the sea above the gravity current at relatively low speed due to the shearing between the landward gravity current and the seaward synoptic wind. Such dynamic conditions may enhance an accumulation of pollutants above coastal industrial areas.

Multiple carbon substrate utilization by bacteria at the sediment–water interface: seasonal patterns in a stratified eutrophic reservoir

Sediment–water interfaces (SWIs) in lakes and reservoirs harbor diverse assemblages of heterotrophic bacteria that influence organic matter cycling. Seasonal reservoir mixing and stratification affect the rate and type of dissolved organic carbon (DOC) utilized by SWI bacteria, however these seasonal carbon utilization dynamics remain poorly understood. We conducted an exploratory multi-seasonal study using Biolog EcoPlates to measure SWI bacterial carbon substrate utilization rates (CSURs) and patterns in a monomictic eutrophic reservoir. Principal components analysis (PCA) was used to elucidate seasonal CSUR variation. Carbohydrate utilization was greatest during early (onset of) stratification and also was high during winter mixing. Amino acid utilization was greatest during late (prolonged) stratification. Carboxylic acids had greatest utilization during late stratification, but also had moderate to high utilization during all other seasons. Amines and polymers exhibited moderate utilization during all seasons. We related seasonal variation in these CSURs to SWI bacterial abundance, temperature, dissolved oxygen, and redox potential. Collectively these environmental variables accounted for 49.4–62.3% of the total substrate utilization variance by season. Several individual substrate CSURs were significantly (P < 0.05) positively and/or negatively correlated with individual environmental variables, with l-asparagine and putrescine exhibiting significant positive and negative correlations with every measured environmental variable. Lastly, seasonal changes in CSURs corresponded to various changes in SWI bacterial assemblage composition, with highest similarities among early stratification and autumnal overturn assemblages and lowest similarities among late stratification and winter mixing assemblages.

Zooplankton dynamics during autumn circulation in a small, wind-sheltered, Mediterranean lake

Few studies have been undertaken on zooplankton dynamics during the transition from late stratification to early mixing in lakes. The Plankton Ecology Group (PEG) model of plankton seasonality only considers water temperature, edible phytoplankton and fish predation as zooplankton-controlling factors during that period. The water-column edible algal and bacterial fractions, heterotrophic nanoflagellates, ciliates, rotifers, cladocerans and copepods were studied during 93 consecutive days of mixing-transition in a wind-sheltered, Mediterranean lake without planktivorous fish. Abundances of all zooplankton populations except ciliates, Daphnia longispina and copepodites, were related to water temperature. In addition to PEG controlling factors, time series analysis indicated competition between ciliates and Ceriodaphnia reticulata and Polyarthra dolichoptera and D. longispina, predation of Arctodiaptomus salinus on ciliates, herbivory of adult copepods on the 5–20 μm phytoplanktonic size and delayed effects shorter than one week. The remaining zooplankton populations did not show any edible size preference. Ceriodaphnia reticulata showed evidence of herbivory on bacteria and picoplankton and competition with Hexarthra fennica in shorter periods. Phyto- and zooplankton production were decoupled. These results, and others from warmer periods and latitudes, suggest that the current model of lake plankton seasonality must be reviewed.

Seasonal dynamics of the phytoplankton community and cyanobacterial dominance in a eutrophic crater lake in Los Tuxtlas, Mexico

:In Los Tuxtlas, Mexico, there are numerous deep crater lakes in varying trophic states. In a warm eutrophic monomictic lake of the region we analyzed changes in the phytoplankton community and how they relate to water quality. During the periods of circulation, early and late stratification, there was an increase in species diversity marked by an increase in the abundance of many species of diatoms and chlorophytes. These periods were characterized by low transparency, high specific conductance, and high concentrations of ammonium and dissolved silica. Microcystis aeruginosa dominated during the entire stratification period, when both diversity and succession rates were low. Stratification was characterized by high temperatures (> 30 °C), low transparency, high pH, and high concentrations of soluble reactive phosphorus and nitrate. Thus, in Lake Chalchoapan, physical changes associated with stratification, circulation, and tropical seasonality determine the succession dynamics of the phytoplankton community.

The phytoplankton community as a bioindicator of health conditions of Atezca Lake, Mexico

Atezca Lake is located in a mountainous zone of North–eastern Mexico. It is a small subtropical monomictic system with a winter circulation period, and stratification in spring and summer. The lake was first studied monthly during two annual cycles in 1981 and 1983; biannual sampling was then conducted in the stratification and circulation periods for 10 years (1984–1994) in order to determinate the seasonal and annual succesional patterns of the phytoplankton community.Thermal behaviour was found to influence both physical and chemical features. A high concentration of nitrates and phosphates occurred at the beginning of the stratification period and decreased toward late stratification. During the overturn period these nutrients increased. These dynamics affected the phytoplankton assemblage because bacillariophyceans and chlorophyceans were dominant in early stratification, dinophyceans and cyanophyceans in late stratification and bacillariophyceans and chrysophyceans were abundant at circulation.The long term changes showed an increase in the thickness of the anoxic hypolimnion; surface nitrates also diminished, whereas phosphates tended to increase. From 1981 to 1986 the phytoplankton community was dominated by chlorphyceans; however from 1990 to 1994 they were replaced by cyanophyceans, especially Microcystis aeruginosa. The results indicate that the lake has shown severe signs of eutrophication in recent years. The prospects for recovery depend upon sewage input control, hyplomnetic discharges and conservation of the forest in the catchment area.

The phytoplankton community as a bioindicator of health conditions of Atezca Lake, Mexico

Abstract Atezca Lake is located in a mountainous zone of North–eastern Mexico. It is a small subtropical monomictic system with a winter circulation period, and stratification in spring and summer. The lake was first studied monthly during two annual cycles in 1981 and 1983; biannual sampling was then conducted in the stratification and circulation periods for 10 years (1984–1994) in order to determinate the seasonal and annual successional patterns of the phytoplankton community. Thermal behaviour was found to influence both physical and chemical features. A high concentration of nitrates and phosphates occurred at the beginning of the stratification period and decreased toward late stratification. During the overturn period these nutrients increased. These dynamics affected the phytoplankton assemblage because bacillariophyceans and chlorophyceans were dominant in early stratification, dinophyceans and cyanophyceans in late stratification and bacillariophyceans and chrysophyceans were abundant at circulation. The long term changes showed an increase in the thickness of the anoxic hypolimnion; surface nitrates also diminished, whereas phosphates tended to increase. From 1981 to 1986 the phytoplankton community was dominated by chlorphyceans; however from 1990 to 1994 they were replaced by cyanophyceans, especially Microcystis aeruginosa. The results indicate that the lake has shown severe signs of eutrophication in recent years. The prospects for recovery depend upon sewage input control, hyplomnetic discharges and conservation of the forest in the catchment area.

Planktonic Protozoa in Lakes Huron and Michigan: Seasonal Abundance and Composition of Ciliates and Dinoflagellates

The abundance and biomass of surface (5 m) and deep (30–45 m) ciliate and dinoflagellate protozoa in the offshore waters of Lakes Huron and Michigan were determined from December 1986 to November 1987. Protozoan (ciliates and dinoflagellates) abundance (4 to 15 cells·mL −1) and biomass (13–140 μg·L −1, wet wt.) were comparable between lakes and similar to those reported from other oligotrophic environments. On average, ciliates comprised the majority of protozoan abundance (80%) and biomass (73%). The mean size (ESD) of these communities was small (20.6 μm) due to the numerical dominance of small choreotrichs, oligotrichs, and species of Gymnodinium. Total biomass in both lakes peaked during late June-July and again during the October-November period. These seasonal changes in biomass were accompanied by species replacements: tintinnids, strobilids, oligotrichs, and Gymnodinium species were abundant in the spring isothermal period, shifting to oligotrich dominance during summer stratification (May–July); a more diverse assemblage followed during late stratification (October–November) in which haptorids, prorodontids, and Peridinium species became more important. Deep and surface communities were comparable in terms of abundance and biomass, although deep community biomass decreased as stratification intensified. Because the biomass of ciliates alone represents approximately 30% of crustacean zooplankton biomass, protozoa may be more important grazers than once thought.

Organic nitrogen mineralization and substrate limitation of bacteria in Lake Michigan

Labile organic nitrogen mineralization and the apparent degree of bacterial substrate limitation were examined to consider seasonal relationships between substrate availability and bacterial activity in Lake Michigan. Accumulation rates of ammonium nitrogen in amino acid fortified and unfortified samples of epilimnetic Lake Michigan water, incubated in the dark, provided reasonable estimates of potential and actual rates of organic nitrogen mineralization. The labile organic nitrogen demand (LOND), defined as the difference between these respective rates, provided an index of heterotrophic potential. LOND ranged from ∼1–3 ng‐atoms N liter h−1 (during May–June and November) to 3–9 ng‐atoms N liter h−1(during July–October) as compared to actual organic nitrogen mineralization rates of &lt; 1 ng‐atom N liter h−1 in some unfortified samples. The high LOND, relative to actual turnover, observed in late summer is consistent with the hypothesis that growth rates of epilimnetic Lake Michigan bacteria are strongly limited by organic substrate during late stratification.

A Radiotracer Study of Phosphorus Cycling in a Eutrophic Canadian Shield Lake, Lake 227, Northwestern Ontario

Phosphoric acid labelled with32P was added to the epilimnion of experimentally eutrophied Lake 227, northwestern Ontario, in August 1978 to trace the lake's phosphorus cycle during late stratification and fall overturn. Radiophosphate was incorporated into bacteria and microphytoplankton (&lt;10 μm in diameter) within minutes of introduction. Thereafter, the bulk of32P exchange was between the microplankton, which typically held &gt;90% of32P, dissolved phosphate, and a mobile subcompartment of "dissolved organic" phosphorus. Phytoplankton &gt;10 μm in diameter and zooplankton acquired32P label very slowly and contained &lt;5% of the radiotracer. In addition, about half of dissolved organic phosphorus failed to acquire a32P label over 17 d of incubation, indicating that this phosphorus may be functionally inert, at least at the time scale of biological phosphorus exchange. Movement of32P to the littoral zone of Lake 227 was much slower than in lakes with well-developed macrophyte communities, but losses to deep sediments were similar, about 2% of32P per day. Particles settled through the hypolimnion at a modal velocity of 28 cm∙d−1, losing 10% or less of their32P during descent. Most of the released phosphorus was reincorporated into particles and sedimented. Less than 1% of the32P that entered the hypolimnion in particles between August and October remained in the hypolimnion in late October.32P release from Lake 227′s hypolimnetic sediments during late summer and fall of 1978 was &lt;5% of the32P that sedimented.