Estimation of reference conditions for phytoplankton in a naturally eutrophic shallow lake

Classification of waters using biological quality elements and determination of the degree of deviation from reference levels is a key issue in the Water Framework Directive of EU. Lakes in reference conditions with sufficient biological data are available for several boreal lake types with the exception of naturally eutrophic lakes. An empirical approach is one alternative for estimating the reference conditions of such lakes. We used the water transparency of the naturally eutrophic Lake Tuusulanjärvi recorded in August in the early 1910s to estimate reference values for phytoplankton biomass and chlorophyll a concentrations. Three phytoplankton samples during August 2000–2001 corresponded to the estimated reference values for total biomass (<5.6 mg l^?1) and chlorophyll a (<28 μg l^?1), as did the simultaneous Secchi depths. The phytoplankton assemblage in these samples with 24 eutrophy indicators (17% of the total taxa number) corresponded in general the species list from the early 1900s, which as such could be regarded as reference assemblage. Furthermore, in August 2000, 3 years after intensive fish removal a prominent decrease in cyanobacterial biomass and toxin concentration was observed. The costs of the measures and studies in Lake Tuusulanjärvi during 1989–2003 have been approximately 2.5 million euros.     

Refuge availability and sequence of predators determine the seasonal succession of crustacean zooplankton in a clay-turbid lake

The contribution of predators and abiotic factors to the regulation of the biomass and seasonal succession of crustacean zooplankton was studied in Lake Rehtijärvi (southern Finland). Field data in combination with bioenergetics modeling indicated that invertebrate planktivory by Chaoborus depressed cladoceran populations during early summer. In particular, bosminids that generally form the spring biomass peak of cladocerans in stratified temperate lakes did not appear in the samples until July. In July, predation pressure by chaoborids was relaxed due to their emergence period and cladoceran population growth appeared to be limited by predation by planktivorous fish. The effect of fish predation was amplified by reduced refuge availability for cladocerans. The concentration of dissolved oxygen below the epilimnion was depleted, forcing cladocerans to move upward to less turbid and thus more dangerous water layers. The effect of size selective predation by fish resulted in reduced mean size of cladocerans during the period when refuge thickness (thickness of the water layer with oxygen concentration <1 mg l^?1 and water turbidity >30 NTU) was lowest. The results confirmed that in clay-turbid lakes, invertebrate predators could be the main regulators of herbivorous zooplankton even when cyprinid fish are abundant.     

Grazing effects of a freshwater bivalve (Corbicula leana Prime) and large zooplankton on phytoplankton communities in two Korean lakes

This study examined the effects of a freshwater filter feeding bivalve (Corbicula leana Prime) and large zooplankton (>200 μm, mostly cladocerans and copepods) on the phytoplankton communities in two lakes with contrasting trophic conditions. A controlled experiment was conducted with four treatments (control, zooplankton addition, mussel addition, and both zooplankton and mussel addition), and each established in duplicate 10-l chambers. In both lakes there were significant effects of mussel grazing on phytoplankton density and biomass. The effects were greater in mesotrophic Lake Soyang than in hypertrophic Lake Ilgam. Effects of zooplankton grazing did not differ between these lakes, and zooplankton effects on phytoplankton were much less than the effects of mussels. Although mussels exerted a varying effect on phytoplankton according to their size, mussels reduced densities of almost all phytoplankton taxa. Total mean filtering rate (FR) of mussels in Lake Soyang was significantly greater than that in Lake Ilgam (p=0.002, n=5). Carbon fluxes from phytoplankton to mussels (977–2,379 μgC l^?1d^?1) and to zooplankton (76–264 μgC l^?1 d^?1) were always greater in Lake Ilgam due to the greater phytoplankton biomass (p<0.01, n=6). Based on the C-flux to biomass ratios, the mussels consumed 170–754% (avg. 412%) of phytoplankton standing stock in Lake Soyang, and 38–164% (avg. 106%) in Lake Ilgam per day. The C-flux to biomass ratio for mussels within each lake was much greater than for large zooplankton. Mussels reduced total phosphorus concentration by 5–34%, while increasing phosphate by 30–55% relative to the control. Total nitrogen also was reduced (by 9–25%), but there was no noticeable change in nitrate among treatments. The high consumption rate of phytoplankton by Corbicula leana even in a very eutrophic lake suggests that this mussel could affect planktonic and benthic food web structure and function by preferential feeding on small seston and by nutrient recycling. Control of mussel biomass therefore might be an effective tool for management of water quality in shallow eutrophic lakes and reservoirs in Korea.     

Phytoplankton blooms under dim and cold conditions in freshwater lakes of East Antarctica

The seasonal variations of limnological (water temperature, light availability, turbidity, and chlorophyll a concentration) parameters were recorded continuously from January 2004 to February 2005 at two freshwater lakes: Oyako-ike and Hotoke-ike, Sôya Coast, East Antarctica. Water was in a liquid phase throughout the year, with temperatures ranging from 0 to 10°C. The maximum photosynthetically active radiation in Lake Oyako-ike was 23.16 mol m^?2 day^?1 (at 3.8 m) and Hotoke-ike was 53.01 mol m^?2 day^?1 (at 2.2 m) in summer, and chlorophyll a concentration ranged from ca. 0.5 to 2.5 μg L^?1 (Oyako-ike) and from ca. 0.1 to 0.8 μg L^?1 (Hotoke-ike) during the study period. Increase in chlorophyll a fluorescence occurred under dim-light conditions when the lakes were covered with ice in spring and autumn, but the signals were minimum in ice-free summer in both the lakes. During spring and summer, as a result of decreasing snow cover, the chlorophyll a concentration similarly decreased when PAR was relatively high, following periods of heavy winds. The autumnal and spring increase occurred under different PAR levels (ca. 20-fold and 90-fold stronger, respectively, in autumn in both the lakes). Differences in the autumn and spring increases suggest that the spring algal community is more shade-adapted than the autumn algal community. Antarctic phytoplankton appears especially adapted to low-light levels and inhibited by strong light regimes.     

Precipitation-Induced Alternative Regime Switches in Shallow Lakes of the Boreal Plains (Alberta,Canada)

This study focused on unraveling the natural mechanism for the frequent shifts in alternative regimes in pristine shallow lakes of the Boreal Plains, Alberta, Canada. The lakes tend to be clear and dominated by submerged aquatic vegetation (SAV) or turbid and dominated by phytoplankton. We report on the inter-annual response of 23 lakes from 2001 to 2007. We explore the effect of fluctuations in annual precipitation on the lake response including water depth, total phosphorus (TP) concentration, turbidity, phytoplankton biomass, SAV biomass, and the proportion of clear and turbid lakes. The regime switches appear driven by the transient dynamics of phytoplankton, and dilution of nutrients, phytoplankton biomass, and turbidity during wet years, and evapoconcentration during dry years. Increased precipitation was correlated with decreased phytoplankton biomass, TP concentration, chloride concentration, and turbidity. In 2005, the wettest year, no phytoplankton-dominated lakes were observed. During the driest year (2002), the phytoplankton-dominant regime (>18 μg chl-a L^?1) occurred in 22% of lakes, which was higher than the study period average. SAV biomass was not directly affected by precipitation, but was negatively associated with phytoplankton biomass and positively associated with the previous year’s SAV growth. SAV biomass was carried over from year-to-year, and the occurrence of SAV-dominated (>25% cover) lakes was significantly higher in 2007 (90%) following 3 years of high precipitation levels.     

First records of primary producers of epiglacial and supraglacial lakes in western Dronning Maud Land, Antarctica

Epiglacial and supraglacial lakes are characteristic lake types in Antarctica, and regardless of their mostly seasonal existence and ultraoligotrophy, some lakes have a relatively diverse microbial community. The results of water chemistry and phytoplankton, based on basic limnological methods, from five epiglacial and two supraglacial seasonal lakes are presented from western Dronning Maud Land, an area where only physical studies have been previously carried out. Electric conductivity varied mostly between 0.1 and 10 mS m^?1 (25 °C), phosphorus concentration was <5 mg m^?3, and nitrogen concentration was <300 mg m^?3 except in some shore areas, and water pH ranged from 6 to 11. Low phytoplankton biomasses (in most cases <10 mg m^?3) supported the ultraoligotrophic status of the lakes. Phytoplankton was found from both types of lakes, but less was found from supraglacial lakes. The charophyte Mesotaenium cf. berggrenii dominated the supraglacial lakes, while cyanoprokaryotes such as Gloeocapsopsis cf. magma, Planktothrix prolifica/rubescens, Nostoc cf. sphaericum, Cyanothece sp. and Phormidium sp. dominated the biomass in some epiglacial lakes. Chrysophytes (e.g. Pseudopedinella-type flagellates) were observed in both types of lakes, and they were occasionally dominant. The green alga Botryococcus braunii, some diatoms (Cyclotella sp., Diatoma tenuis, Luticola muticopsis), and non-planktonic microalgal colonies visible to the eye (incl. the cyanoprokaryote Nostoc commune) were also found. Signs of a living ecosystem with a food web were observed in one epiglacial lake, but not elsewhere, which indicates extreme circumstances in the Antarctic seasonal lakes. Altogether, only some 25 taxa were discovered.     

Ontogenetic changes in the predator–prey interactions between threadsail filefish and moon jellyfish

The global climate change may lead to more extreme climate events such as severe flooding creating excessive pulse-loading of nutrients, including nitrogen (N), to freshwaters. We conducted a 3-month mesocosm study to investigate the responses of phytoplankton, zooplankton and Vallisneria spinulosa to different N loading patterns using weekly and monthly additions of in total 14 g N m^?2 month^?1 during the first 2 months. The monthly additions led to higher phytoplankton chlorophyll a and total phytoplankton biomass than at ambient conditions as well as lower leaf biomass and a smaller ramet number of V. spinulosa. Moreover, the biomass of cyanobacteria was higher during summer (August) in the monthly treatments than those with weekly or no additions. However, the biomass of plankton and macrophytes did not differ among the N treatments at the end of the experiment, 1 month after the termination of N addition. We conclude that by stimulating the growth of phytoplankton (cyanobacteria) and reducing the growth of submerged macrophytes, short-term extreme N loading may have significant effects on shallow nutrient-rich lakes and that the lakes may show fast recovery if they are not close to the threshold of a regime shift from a clear to a turbid state.     

Substantial differences in littoral fish community structure and dynamics in subtropical and temperate shallow lakes

1. Fish play a key role in the functioning of temperate shallow lakes by affecting nutrient exchange among habitats as well as lake trophic structure and dynamics. These processes are, in turn, strongly influenced by the abundance of submerged macrophytes, because piscivorous fish are often abundant at high macrophyte density. Whether this applies to warmer climates as well is virtually unknown. 2. To compare fish community structure and dynamics in plant beds between subtropical and temperate shallow lakes we conducted experiments with artificial submerged and free‐floating plant beds in a set of 10 shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N), paired along a gradient of limnological characteristics. 3. The differences between regions were more pronounced than differences attributable to trophic state. The subtropical littoral fish communities were characterised by higher species richness, higher densities, higher biomass, higher trophic diversity (with predominance of omnivores and lack of true piscivores) and smaller body size than in the comparable temperate lakes. On average, fish densities were 93 ind. m⁻² (±10 SE) in the subtropical and 10 ind. m⁻² (±2 SE) in the temperate lakes. We found a twofold higher total fish biomass per unit of total phosphorus in the subtropical than in the temperate lakes, and as fish size is smaller in the former, the implication is that more energy reaches the littoral zone fish community of the warmer lakes. 4. Plant architecture affected the spatial distribution of fish within each climate zone. Thus, in the temperate zone fish exhibited higher densities among the artificial free‐floating plants while subtropical fish were denser in the artificial submerged plant beds. These patterns appeared in most lakes, regardless of water turbidity or trophic state. 5. The subtropical littoral fish communities resembled the fish communities typically occurring in temperate eutrophic and hypertrophic lakes. Our results add to the growing evidence that climate warming may lead to more complex and omnivory‐dominated food webs and higher density and dominance of smaller‐sized fish. This type of community structure may lead to a weakening of the trophic cascading effects commonly observed in temperate shallow lakes and a higher risk of eutrophication.     

Seasonal variation in light utilisation,biomass production and nutrient removal by wastewater microalgae in a full-scale high-rate algal pond

There has been renewed interest in the combined use of high-rate algal ponds (HRAP) for wastewater treatment and biofuel production. Successful wastewater treatment requires year-round efficient nutrient removal while high microalgal biomass yields are required to make biofuel production cost-effective. This paper investigates the year-round performance of microalgae in a 5-ha demonstration HRAP system treating primary settled wastewater in Christchurch, New Zealand. Microalgal performance was measured in terms of biomass production, nutrient removal efficiency, light absorption and photosynthetic potential on seasonal timescales. Retention time-corrected microalgal biomass (chlorophyll a) varied seasonally, being lowest in autumn and winter (287 and 364 mg m^?3day^?1, respectively) and highest in summer (703 mg m^?3day^?1), while the conversion efficiency of light to biomass was greatest in winter (0.39 mg Chl- a per μmol) and lowest in early summer (0.08 mg Chl- a per μmol). The percentage of ammonium (NH₄–N) removed was highest in spring (79 %) and summer (77 %) and lowest in autumn (47 %) and winter (53 %), while the efficiency of NH₄–N removal per unit biomass was highest in autumn and summer and lowest in winter and spring. Chlorophyll-specific light absorption per unit biomass decreased as total chlorophyll increased, partially due to the package effect, particularly in summer. The proportional increase in the maximum electron transport rate from winter to summer was significantly lower than the proportional increase in the mean light intensity of the water column. We concluded that microalgal growth and nutrient assimilation was constrained in spring and summer and carbon limitation may be the likely cause.     

Enhancement of phenylpropanoid enzymes and lignin in Phalaenopsis orchid and their influence on plant acclimatisation at different levels of photosynthetic photon flux

Effects of three levels of photosynthetic photon flux (PPF: 60, 160 and 300 μmol m⁻²s⁻¹) were investigated in one-month-old Phalaenopsis plantlets acclimatised ex vitro. Optimal growth, chlorophyll and carotenoid concentations, and a high carotenoid:chlorophyll a ratio were obtained at 160 μmol m⁻²s⁻¹, while net CO₂ assimilation (A), stomatal conductance (g), transpiration rate (E) and leaf temperature peaked at 300 μmol m⁻²s⁻¹, indicating the ability of the plants to grow ex vitro. Adverse effects of the highest PPF were reflected in loss of chlorophyll, biomass, non-protein thiol and cysteine, but increased proline. After acclimatisation, glucose-6-phosphate dehydrogenase, shikimate dehydrogenase, phenylalanine ammonia-lyase (PAL) and cinnamyl alcohol dehydrogenase (CAD) increased, as did lignin. Peroxidases (POD), which play an important role in lignin synthesis, were induced in acclimatised plants. Polyphenol oxidase (PPO) and β-glucosidase (β-GS) activities increased to a maximum in acclimatised plants at 300 μmol m⁻²s⁻¹. A positive correlation between PAL, CAD activity and lignin concentration was observed, especially at 160 and 300 μmol m⁻²s⁻¹. The study concludes that enhancement of lignin biosynthesis probably not only adds rigidity to plant cell walls but also induces defence against radiation stress. A PPF of 160 μmol m⁻²s⁻¹was suitable for acclimatisation when plants were transferred from in vitro conditions.     

Phytoplankton and its dynamics in two tropical lakes: a tropical and temperate zone comparison

Temporal patterns of phytoplankton biomass and community structure are described for two Kenyan lakes and subsequently compared with patterns reported in other tropical and temperate lakes. Lake Naivasha had a lower and more seasonally variable (10×) biomass, with a seasonal shift between diatoms and blue-greens, while the L. Oloidien biomass was less variable (3.7×) and dominated by blue-greens. Biomass and chlorophyll a were strongly correlated and in turn were coupled to the level of total phosphorus. A total of 143 and 94 taxa were described for L. Naivasha and L. Oloidien, respectively.The comparative analysis showed: a) a paucity of exclusively tropical species; b) that more than 30 percent of the species in two highly saline Kenyan lakes were also present in the two freshwater lakes; c) no evidence for a postulated decline of phytoplankton species abundance with latitude from the temperate zone to the tropics; d) that the low fraction of chrysophyte biomass in tropical lakes is a function of trophy rather than of latitude; e) that the fraction of chlorophyte biomass in tropical lakes is generally higher than in temperate lakes; f) that the proportion of nannoplankton in the two Kenyan freshwater lakes is not different from that in temperate lakes of the same trophy; g) that seasonal or annual biomass oscillations in the tropics are not systematically lower than in the temperate zone; h) evidence for large inter-year difference in the max.:min. biomass ratio in the only tropical lake (L. Naivasha) for which such data are available; i) that an average biomass ratio appears predictable for tropical lakes from the proportion of the sediment surface in contact with epilimnetic water. Overall, no evidence was found that the freshwater tropical phytoplankton composition or dynamics differ in any fundamental fashion from that observed in the temperate lakes during the summer.Contribution number 147 of the Limnology Research Centre, McGill University.     

Ecophysiology and genetic structure of polar versus temperate populations of the lichen Cetraria aculeata

We studied polar and temperate samples of the lichen Cetraria aculeata to investigate whether genetical differences between photobionts are correlated with physiological properties of the lichen holobiont. Net photosynthesis and dark respiration (DR) at different temperatures (from 0 to 30 °C) and photon flux densities (from 0 to 1,200 μmol m^?2 s^?1) were studied for four populations of Cetraria aculeata. Samples were collected from maritime Antarctica, Svalbard, Germany and Spain, representing different climatic situations. Sequencing of the photobiont showed that the investigated samples fall in the polar and temperate clade described in Fernández-Mendoza et al. (Mol Ecol 20:1208–1232, 2011). Lichens with photobionts from these clades differ in their temperature optimum for photosynthesis, maximal net photosynthesis, maximal DR and chlorophyll content. Maximal net photosynthesis was much lower in Antarctica and Svalbard than in Germany and Spain. The difference was smaller when rates were expressed by chlorophyll content. The same is true for the temperature optima of polar (11 °C) and temperate (15 and 17 °C) lichens. Our results indicate that lichen mycobionts may adapt or acclimate to local environmental conditions either by selecting algae from regional pools or by regulating algal cell numbers (chlorophyll content) within the thallus.     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. This synthesis examines 35 long‐term (5–35 years, mean: 16 years) lake re‐oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in‐lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in‐lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L^?1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Patterns in Abundance,Cell Size and Pigment Content of Aerobic Anoxygenic Phototrophic Bacteria along Environmental Gradients in Northern Lakes

There is now evidence that aerobic anoxygenic phototrophic (AAP) bacteria are widespread across aquatic systems, yet the factors that determine their abundance and activity are still not well understood, particularly in freshwaters. Here we describe the patterns in AAP abundance, cell size and pigment content across wide environmental gradients in 43 temperate and boreal lakes of Québec. AAP bacterial abundance varied from 1.51 to 5.49 x 10⁵ cells mL^-1, representing <1 to 37% of total bacterial abundance. AAP bacteria were present year-round, including the ice-cover period, but their abundance relative to total bacterial abundance was significantly lower in winter than in summer (2.6% and 7.7%, respectively). AAP bacterial cells were on average two-fold larger than the average bacterial cell size, thus AAP cells made a greater relative contribution to biomass than to abundance. Bacteriochlorophyll a (BChla) concentration varied widely across lakes, and was not related to AAP bacterial abundance, suggesting a large intrinsic variability in the cellular pigment content. Absolute and relative AAP bacterial abundance increased with dissolved organic carbon (DOC), whereas cell-specific BChla content was negatively related to chlorophyll a (Chla). As a result, both the contribution of AAP bacteria to total prokaryotic abundance, and the cell-specific BChla pigment content were positively correlated with the DOC:Chla ratio, both peaking in highly colored, low-chlorophyll lakes. Our results suggest that photoheterotrophy might represent a significant ecological advantage in highly colored, low-chlorophyll lakes, where DOC pool is chemically and structurally more complex.     

Low Levels of Allochthony in Consumers Across Three High-Elevation Lake Types

The integration of lakes into watershed-scale energy budgets remains a major goal of aquatic ecosystem ecology. However, this enterprise has focused almost exclusively on temperate and boreal systems and on zooplankton as representatives of system-wide energy flow. We investigated the proportion of consumer biomass derived from terrestrial sources, allochthony, in three classes of high-elevation lakes—alpine, large and small montane—of varying geometry and watershed ecosystem development, and across five taxa, including macrobenthic invertebrates and fish. We used stable isotopes of carbon (¹³C:¹²C), nitrogen (¹⁵N:¹⁴N), and hydrogen (²H:¹H) to fit a modular Bayesian mixing model, which estimated proportional assimilation of phytoplankton, algal periphyton, and terrestrial organic matter by each consumer. The stable isotope analysis was supplemented with a comparison of fatty acid profiles between consumers and producers, and with a Daphnia magna rearing study involving aquatic and terrestrial nutrient sources. The most probable estimate of allochthony across consumer taxa was 41% in small montane lakes (< 0.1 ha) with high terrestrial C loading. For large montane (3–11 ha) and alpine lakes (0.8–3 ha), with substantially less terrestrial influence, allochthony was just 4 and 3%, respectively. Allochthony was also lower on average for benthic grazers than for pelagic consumers. Our results corroborate previous findings that lake size, depth, and light penetration are dominant physical controls on allochthony, but add that it sharply declines at high elevation due to changes in terrestrial primary production near or above tree line.     

Winter ecology of shallow lakes: strongest effect of fish on water clarity at high nutrient levels

While the structuring role of fish in lakes is well studied for the summer season in North temperate lakes, little is known about their role in winter when fish activity and light irradiance potentially are lower. This is unfortunate as the progressing climate change may have strong effects on lake winter temperature and possibly on trophic dynamics too. We conducted an enclosure experiment with and without the presence of fish throughout winter in two shallow lakes with contrasting phosphorus concentrations. In hypertrophic Lake Søbygård, absence of fish led to higher biomass of zooplankton, higher grazing potential (zooplankton:phytoplankton ratio) and, accordingly, lower biomass of phytoplankton and chlorophyll a (Chl a), while the concentrations of total nitrogen (TN), total phosphorus (TP), oxygen and pH decreased. The average size of egg-bearing Daphnia and Bosmina and the minimum size of egg-bearing specimens of the two genera rose. In the less eutrophic Lake Stigsholm, zooplankton and their grazing potential were also markedly affected by fish. However, the decrease in Chl a was slight, and phytoplankton biovolume, pH and the oxygen concentration were not affected. TN was higher when fish were absent. Our results indicate that: (i) there is a notable effect of fish on zooplankton community structure and size during winter in both eutrophic and hypertrophic North temperate lakes, (ii) Chl a can be high in winter in such lakes, despite low light irradiance, if fish are abundant, and (iii) the cascading effects on phytoplankton and nutrients in winter may be more pronounced in hypertrophic lakes. Climate warming supposedly leading to reduced winter mortality and dominance of small fish may enhance the risk of turbid state conditions in nutrient-enriched shallow lakes, not only during the summer season, but also during winter.     

Spatial distribution, biomass and population dynamics of Mysis relicta in Lake Michigan

The abundance, biomass, and life history traits of Mysis relicta were evaluated in the spring, summer, and autumn of 2000 at 8 nearshore (45 m) and 8 offshore (75–110 m) stations throughout central and southern Lake Michigan. Abundance was also evaluated on a smaller scale during June 2000 at 4 nearshore and 4 offshore stations in southeastern Lake Michigan. For large-scale sampling, the abundance of M. relicta did not differ among locations in the spring. In the summer and autumn the abundance of M. relicta was similar among offshore stations with the exception of one station each season; for nearshore stations, abundance was generally highest off Pentwater, Michigan. The abundance of mysids was not consistently high for central or southern basin sites, although overall biomass was higher in the southern basin each season. Abundance of Mysis was positively correlated with bottom depth, but not with bottom water temperature, surface water temperature, or mean chlorophyll concentration. Within the smaller region in southeast Lake Michigan, the abundance of M. relicta differed among locations for both nearshore and offshore stations. Brood size and size of reproductive females did not differ among lake wide locations, but the proportion of females with broods and the size distribution of M. relicta did.     

Temporal variability in egg production rates of Acartia tonsa Dana in Long Island Sound

The relationship between week-by-week variations in the in situ egg production rates of Acartia tonsa Dana and changes in chlorophyll concentration in several size fractions was investigated by incubating adult females in natural sea water for 24-h periods. Our results indicate that the egg production of A. tonsa in Long Island Sound was better related to the 10 μm chlorophyll size fraction than to the total chlorophyll concentration. The < 10 μm size fraction comprised the greatest percentage of the chlorophyll during July and August when the water column was stratified. Egg production rates were lowest (8.7 eggs · female⁻¹ · day⁻¹) in early August when less than 0.5 μg chlorophyll 1 ⁻¹ was observed in the 10 μm chlorophyll a size fraction. Following destratification in late August, the “fall” diatom bloom occurred and egg production rates increased to the maximum observed rate of 56.6 eggs · female⁻¹ · day⁻¹. At this time, the concentration of the 10μm chlorophyll size fraction was 5.5 μg 1⁻¹. Maximum egg production rates were observed at chlorophyll concentrations as low as 0.8 μg 1⁻¹ in the 10 μm size fraction.     

Maximum growing depth of submerged macrophytes in European lakes

Submerged macrophytes are important elements for the structure and functioning of lake ecosystems. In this study, we used chemical and maximum colonisation depth (C_max) data from 12 European countries in order to investigate how suitable C_max may describe the impact by eutrophication. The analyses include data from 757 lakes and 919 lake years covering oligotrophic to eutrophic lakes. Overall, C_max was closely related to Secchi depth (R ² = 0.58) and less closely to chlorophyll a (R ² = 0.31), TP (R ² = 0.31) and total nitrogen, TN (R ² = 0.24). The low coefficients of determination between C_max and nutrient concentrations suggest that other response factors than nutrient-phytoplankton-light conditions are important for C_max and that it will be difficult to establish strong relationships between external nutrient loading and C_max. Yearly monitoring for 13–16 years in eight Danish lakes showed considerable year-to-year variability in C_max, which for the individual lakes only related weakly to changes in Secchi depth. The use of C_max as an eutrophication indicator is especially relevant in not very shallow lakes (maximum depth >4–5 m), not too turbid lakes (C_max >1 m) and not very humic lakes (colour <60 mg Pt/l).     

The plankton community of Lake Matano: factors regulating plankton composition and relative abundance in an ancient,tropical lake of Indonesia

Recent evidence reveals that food webs within the Malili Lakes, Sulawesi, Indonesia, support community assemblages that are made up primarily of endemic species. It has been suggested that many of the species radiations, as well as the paucity of cosmopolitan species in the lakes, are related to resource limitation. In order to substantiate the possibility that resource limitation is playing such an important role, a study of the phytoplankton and zooplankton communities of Lake Matano was implemented between 2000 and 2004. We determined species diversity, relative abundances, size ranges, and total biomass for the phytoplankton and zooplankton, including the distribution of ovigerous individuals throughout the epilimnion of Lake Matano in three field seasons. The phytoplankton community exhibited very low biomass (<15 μg l^?1) and species richness was depressed. The zooplankton assemblage was also limited in biomass (2.5 mg l^?1) and consisted only of three taxa including the endemic calanoid Eodiaptomus wolterecki var. matanensis, the endemic cyclopoid, Tropocyclops matanensis and the rotifer Horaella brehmi. Zooplankton were very small (<600 μm body length), and spatial habitat partitioning was observed, with Tropocylops being confined to below 80 m, while rotifer and calanoid species were consistently observed above 80 m. Less than 0.1% of the calanoid copepods in each year were egg-bearing, suggesting very low population turnover rates. It was concluded that chemical factors as opposed to physical or biological processes were regulating the observed very low standing crops of phytoplankton which in turn supports a very minimal zooplankton community restricted in both species composition and abundance. As chemical factors are a function of the catchment basin of Lake Matano, it is predicted that resource limitation has long played an important role in shaping the unique endemic assemblages currently observed in the food web of the lake.