Variation in phytoplankton composition and limnological features in a water–water ecotone of the Lower Paraná Basin (Argentina)

1. Structure and dynamics of phytoplankton were studied at six sites in a reach of the Lower Paraná River floodplain, from the main course to a marginal shallow lake, connected through a stream. This system is defined as a water–water ecotone and thus, the aim of this work is to analyse the spatial changes of phytoplankton along the transitional system.
2. Temporal fluctuations in number of species and phytoplankton density were more pronounced in the river than in the shallow lake. Even though most species were ubiquitous, there was a clear segregation of the most abundant populations across the ecotone. Aulacoseira granulata var. granulata and other centric diatoms decreased towards the lentic system.
3. Transparency varied according to river discharge, being higher in the lake during high water phases, while the opposite behaviour was observed during dry periods. These fluctuations were related to precipitation and resuspension processes.
4. Nitrate concentration diminished towards the lake while soluble reactive phosphorus remained fairly homogeneous across the ecotone.     

Transient physicochemical microhabitats facilitate fish survival in inhospitable aquatic plant stands

1.  Aquatic plants add structural complexity to aquatic environments, provide resources for the food web and generally promote diversity and stability of aquatic fauna. However, plants also alter the abiotic properties of the water and at high densities can produce physical and chemical conditions intolerable to fish.
2.  We identified substantial horizontal and vertical, as well as spatial and temporal, variability in oxygen and temperature over micro-scales within macrophyte stands. Areas with suitable dissolved oxygen and temperature occurred close to areas with hypoxic and hyperthermic levels regarded as highly stressful to lethal for fish. Fish densities within the plant stands did not change between dusk and dawn, suggesting that fish did not leave the vegetation when conditions became adverse.
3.  Variability in oxygen and temperature within the plant bed conceivably provided refugia for fish. Portions of the macrophyte bed, particularly the surface, provided suitable conditions during times of the day when oxygen tensions in deeper water were low, but not during times when surface conditions were hyperoxic and hyperthermic. During periods of low oxygen, fish may rise to the surface and either remain there, or use the surface as a corridor to travel until oxygenated pockets are located.
4.  Aquatic plant stands may be thought of as a mosaic landscape composed of patches of transient physicochemical microhabitats. Within this landscape, physicochemical conditions can be precarious and fish may have to continuously change microhabitats to avoid being trapped in unsuitable locations during the daily reshuffling of physical and chemical conditions.     

Variation in growth rates of the zebra mussel, Dreissena polymorpha, within Lake Wawasee

1. Field experiments conducted in Lake Wawasee in 1995 and 1996 measured the response of shell growth of Dreissena polymorpha to environmental gradients.
2. Shell growth decreased with initial shell length in four mussel size classes ranging between 8 and 22 mm, and decreased with depth, with mussels in shallow water (<4 m) having growth rates nearly twice those of mussels in deeper water (4–7 m).
3. Growth occurred early in the spring–summer period (May–June) with relatively little shell added later in the summer (July–September), and varied significantly among sites within Lake Wawasee, but not between the 2 years of this study.
4. Rank order of sites was consistent for both years implying that environmental conditions responsible for variation in shell growth were stable within Lake Wawasee.
5. Cage design did not have a significant effect on mussel shell growth nor did the distance of growth cages above the bottom (0.5–0.75 m above the bottom versus directly on the bottom).
6. This study demonstrates the sensitivity of adult mussel growth to subtle variation in environmental conditions occurring within and among lakes, with potential consequences for mussel population dynamics and community structure and function.     

Similarity in seasonal flow regimes,not regional environmental classifications explain synchrony in brown trout population dynamics in France

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Temporal and spatial variation in potential and realized growth rates of age‐0 year northern rock sole

The possibility of prey limitations on the growth performance of age‐0 year northern rock sole Lepidopsetta polyxystra was evaluated at three sites along the north‐east coast of Kodiak Island, Alaska, U.S.A., by comparison of observed to potential growth rates. Growth potential was measured in the laboratory across the range of temperatures encountered by this species during the first summer of life. Growth potential ( g _L, mm day⁻¹) increased with water temperature (T) between 2 and 13° C, according to: g _L = 0·0151 + 0·3673·log₁₀(T). There were significant differences in growth rate between the three field sites such that Holiday Beach fish were 7·1 mm longer than Shakmanof Beach fish by mid‐September, with Pillar Creek Cove fish of intermediate size. Temperature differences between sites accounted for less than half of this variation. The remainder may have been related to differences in prey availability among the sites in association with observed differences in sediment characteristics. In addition to the spatial variability, there was significant monthly variation in growth performance. Realized growth rates between July and August were in excess of 85% of potential. Between August and September, however, realized growth fell to 43–71% of potential indicating a decline in conditions for growth. The spatial variation in growth rates was not density‐dependent as the site with the highest fish densities (Holiday Beach) also supported the highest growth rates. The available data indicates that for this subtidal species, interannual variation in growth may be more important than site variation.     

Synchrony in brown trout, Salmo trutta, population dynamics: a 'Moran effect' on early-life stages

Synchrony among populations (i.e. spatial covariation in temporal fluctuations of population size or growth rate) is a common feature to many animals. Both large-scale autocorrelated climatic factors (the 'Moran effect') and dispersal between populations are candidates to explain synchrony, although their relative influence is difficult to assess. Only a few investigations have reported patterns of synchrony among freshwater populations, and even fewer directly related these patterns to an environmental variable. In the present study, we analysed the spatio-temporal patterns of fluctuation of 57 brown trout populations widespread across France, each sampled continuously during 5 years. We compared the respective influence of connectivity and stream distance within basins (i.e. that potentially allow a basin-scale dispersal) and environmental factors (hydrological and air temperature variables, available for 37 sites) on the synchrony of brown trout cohort densities (0+, 1+ and adults). A series of Mantel tests revealed that the degree of synchrony was not related to connectivity or stream distance between sites, suggesting no effect of dispersal at the basin-scale. The degree of synchrony among sites for the 0+ fish was significantly related to the degree of hydrological synchrony (based on high flows during the emergence period). For all three age classes, the synchrony in the temperature patterns did not explain synchrony in trout dynamics. Our results allow us to discuss the respective influence of dispersal and climatic factors on the spatio-temporal patterns of trout dynamics at the basin scale.     

Temporal and spatial variation in growth of juvenile Atlantic salmon

Spatial and temporal variation in length‐at‐age and environmental factors affecting variation in growth rate of juvenile Atlantic salmon Salmo salar were studied using data from a long‐term study in the River Stjørdalselva, central Norway. Mean annual instantaneous growth rate among 1+ and 2+year juvenile Atlantic salmon varied between 0·59 and 1·50 g g⁻¹ year⁻¹ and mean instantaneous daily growth rate of young‐of‐the‐year (YOY) varied between 0·013 and 0·033 g g⁻¹ day⁻¹. Between year variation in growth was larger than the within year intra‐watercourse spatial variation. For YOY and 1+year Atlantic salmon, a major part of the observed between year variation in growth rates was explained by variation in mean daily water discharge and spring temperature. For 2+year juvenile Atlantic salmon, mean daily water discharge and cohort density were the only variables to significantly explain variation in growth rates. A large part of the within water‐course spatial variation could not be explained by temperature variations and juvenile Atlantic salmon in the uppermost areas of the river, experiencing the lowest ambient temperatures during the growth period, displayed the highest growth rates. Within the baselines set by temperature, biotic and abiotic factors connected to water flow regime and variation in food availability are suggested to be a major determinants of the temporal and spatial variation in juvenile Atlantic salmon growth rates.     

Fish, climate and the Gulf Stream: the influence of abiotic factors on the recruitment success of cyprinid fishes in lowland rivers

1. Climatic effects are increasingly being recognised as an important factor causing inter‐annual variability in organism abundances in aquatic and terrestrial ecosystems. This study investigated the relationships between water temperature (cumulative degree‐days >12 °C), river discharge (cumulative discharge‐days above basal discharge rate), the position of the North Wall of the Gulf Stream (NWGS), and the 0+ growth (September mean length) and recruitment success (year‐class strength, YCS) of three species of cyprinid fishes in two contrasting English lowland rivers, using a 21‐year dataset. 2. Contrary to the majority of studies on 0+ fishes, growth in the Yorkshire Ouse was most significantly correlated with river discharge, with water temperature of less importance. By contrast, temperature was more influential than discharge in the River Trent, possibly because of its regulated hydrological regime, although none of the relationships were statistically significant for this river. 3. Year‐class strength of roach (Rutilus rutilus) was positively correlated with the position of the NWGS, and there was evidence of synchrony in recruitment success between rivers, but the relationships were poorer for chub (Leuciscus cephalus) and dace (Leuciscus leuciscus). The strongest relationships between YCS and discharge during specific time periods were for when the fish were in their early (especially larval) developmental stages, although none of the relationships were statistically significant because of inter‐annual variations in river discharge relative to the timing of fish hatching. 4. Fishes are key predators in the majority of aquatic ecosystems and, as such, fluctuations in their abundances can have implications for ecosystem functioning as a whole. This study has demonstrated an underlying influence of broad‐scale climatic effects on the recruitment of riverine fishes, in spite of local variations in biotic and abiotic conditions. The relative importance of various abiotic factors on the recruitment success of riverine cyprinid populations varies spatially and temporally. For example, river discharge is likely to be of relatively greater importance in poorly‐structured rivers or those that are prone to large and rapid fluctuations in flow, while temporal variations occur because of inter‐annual differences in river discharge relative to the timing of fish hatching. Biotic factors may also be important determinants of fish recruitment success, especially in rivers with stable and predictable flow regimes.     

Effect of springtime water temperature on the time of emergence and size of Pteronarcys californica in the Henry's Fork catchment, Idaho, U.S.A.

1. The emergence time of Pteronarcys californica in streams in the Henry's Fork catchment, Idaho, U.S.A. was negatively correlated with mean April water temperature. Emergence was in mid- to late May at sites influenced by groundwater, where April water temperature averaged 7.9 °C. Adults emerged in mid-June in streams receiving run-off from snowmelt (mean April water temperature 5.4 °C). Intermediate emergence times were observed in a regulated section of river where water temperature was influenced, on one bank, by dam release (mean April water temperature 4.5 °C) and, on the other, by a spring-fed tributary stream (mean April water temperature 6.3 °C).
2. During each of the three study years, emergence was earlier on the bank of the regulated section that was warmer during April and May. The mean body length of P. californica exuviae, collected from the warm side of the river, averaged 1.2 mm longer than those collected from the cold side.
3. We tested the effect on emergence of altering springtime water temperature by translocating P. californica in cages from one location to another during April. Individuals moved to sites with higher April water temperature emerged earlier than individuals that remained at the site from which they were collected.     

A model for resolving the plankton paradox: coexistence in open flows

1. Recent developments in the field of chaotic advection in hydrodynamical/environmental flows encourage us to revisit the population dynamics of competing species in open aquatic systems.
2. We assume that these species are in competition for a common limiting resource in open flows with chaotic advection dynamics. As an illustrative example, we consider a time periodic two-dimensional flow of viscous fluid (water) around a cylindrical obstacle.
3. Individuals accumulate along a fractal set in the wake of the cylinder, which acts as a catalyst for the biological reproduction process. While in homogeneous, well mixed environments only one species could survive this competition, coexistence of competitors is typical in our hydrodynamical system.
4. It is shown that a steady state sets in after sufficiently long times. In this state, the relative density of competitors is determined rather by the fractal nature of the spatial distribution of the advected species, and by their initial conditions, than by their competitive abilities. We argue that two factors, the strong chaotic mixing along a fractal set and the boundary layer around the obstacle, are responsible for the coexistence.     

Dynamics of the exotic Daphnia lumholtzii and native macro-zooplankton in a subtropical chain-of-lakes in Florida, U.S.A.

1. Seasonal dynamics of the exotic Daphnia lumholtzii and native macro-zooplankton species were studied for 2 years in six inter-connected lakes in Florida, U.S.A. The lakes ranged widely in pH, colour and trophic status, and were dominated by copepods.
2. All six lakes contained both D. lumholtzii and the native D. ambigua , but the two species did not overlap in time. D. ambigua was dominant in autumn–spring, coinciding with lower water temperature, higher transparency and lower nutrient and chlorophyll a (Chl a ) concentrations than in summer, when D. lumholtzii was dominant.
3. Based on the field observations, temperature optima were 24 °C for D. ambigua and 29 °C for D. lumholtzii , suggesting that temperature plays a role in determining dominance among the daphnids of subtropical Florida lakes.
4.  D. lumholtzii has not displaced native cladocerans but occupies a 'vacant' seasonal niche, unexploited due to the inability of native taxa to tolerate high temperature. Furthermore, D. lumholtzii did not significantly alter algal–zooplankton interactions. There was evidence of top–down control by grazing, but it was primarily attributable to the native taxa.     

Resistance and resilience of ecosystem metabolism in a flood-prone river system

1. Gross primary production (GPP) and ecosystem respiration (ER) were analysed for 18 months in two reaches of the River Thur, a prealpine river in Switzerland. The upper reach at 655 m above sea level (a.s.l.) is bedrock constrained, has a high slope (0.60%) and a catchment area of 126 km2. The lower reach at 370 m a.s.l. has a more extensive hyporheic zone, a lower slope (0.17%) and a catchment of 1696 km2.
2. In both reaches, temporal patterns of stream metabolism reflected the occurrence of bed-moving spates. Average reductions of GPP and ER by spates were 53 and 24% in the upper reach, and 37 and 14% in the lower reach, respectively. The greater resistance of ER than GPP in both reaches shifted the ecosystem metabolism towards heterotrophy (decrease of the ratio of GPP to ER (P/R)) following spates.
3. Recovery of GPP was significantly faster in the lower reach and exhibited distinct seasonal variation (positive correlation with incident light). The differences in stability (both resistance and resilience) between reaches reflected differences in geomorphic settings and disturbance regime.
4. Stepwise regression analysis was used to explore the potential influence of season, disturbance and prevailing environmental conditions on stream metabolism in each reach. Time since spate plus temperature explained 73 and 86% of variation in ER and GPP, respectively, in the upper reach and 55% of variation in ER in the lower reach. Season plus prevailing environmental conditions explained 67% of variation in GPP in the lower reach.
5. To test how the perception of stability may change with increasing scale of observation, the disturbance regimes of 12 sites were compared with the disturbance regime of the entire Thur catchment. The analysis suggests that stream metabolism at the catchment scale is far more resistant to high flow events than at the reach scale.     

Is water temperature an adequate predictor of recruitment success in cyprinid fish populations in lowland rivers?

SUMMARY 1. Higher than average ambient water temperature in the first year of life may be responsible for strong cohorts of adult cyprinid fish. Whilst temperature explains much of the variation in year‐class strength (YCS), however, it is not the only influential factor as high temperature does not inevitably yield strong year‐classes. Furthermore, years in which a strong year‐class is prevalent in one species do not necessarily result in strong year‐classes in other coexisting species, suggesting other biotic and abiotic factors are important in regulating recruitment success. 2. The relationships between water temperature, river discharge, the position of the Gulf Stream, 0‐group fish growth and recruitment success (YCS) were examined in three cyprinid fish species in an English lowland river, using a 15‐year data set. 3. Mean length of 0‐group fish at the end of the summer was positively correlated with water temperature (cumulative degree‐days >12 °C) and negatively correlated with river discharge (cumulative discharge‐days above basal discharge rate). Water temperature was negatively correlated with river discharge. 4. YCS was positively correlated with mean 0‐group fish length at the end of the summer and with the position of the North Wall of the Gulf Stream. 5. ’Critical periods’ (i.e. periods in the first summer of life when fish may be more susceptible to increases in river discharge) were difficult to discern because of interannual variations in river discharge relative to the timing of fish hatching. YCS of roach and chub was most strongly correlated with discharge in the period from June to September inclusive, while YCS of dace was most significantly correlated with discharge in August. 6. River discharge (rather than water temperature) may be the key factor in determining YCS, either directly (through discharge‐induced mortality) or indirectly (via reduced growth at lower water temperatures, discharge‐associated increases in energy expenditure or reduced food availability). It could be that, in effect, water temperature determines potential YCS while discharge determines realised YCS.     

Plankton population dynamics: food web interactions and abiotic constraints

1. In this introduction, I try to follow some developments in plankton ecology, how they have led to current research topics, and how the contributions in this issue of Freshwater Biology are related to these fields of research.
2. Due to several favourable features, such as small size, short generation time and a relatively homogeneous habitat, planktonic organisms remain ideal subjects for theoretical and experimental population ecology.
3. Important current research topics involve: (1) the control of plankton communities by external abiotic factors; (2) bottom-up (limitation by resources) and top-down (control by predators) effects in the food web; (3) the importance of dormant resting stages and benthic–pelagic coupling in plankton dynamics; (4) costs and benefits of the mixotrophic strategy, i.e. the ability to combine a phototrophic and a phagotrophic mode of nutrition.     

Spatial synchrony in red grouse population dynamics

Red grouse Lagopus lagopus scoticus populations exhibit unstable dynamics that are often characterised by regular periodic fluctuations in abundance. Time-series' of grouse harvesting records collected from 287 management units (moors) across Scotland, England and Wales were analysed to investigate the broad scale patterns of synchrony in these fluctuations. Estimation of the spatial autocorrelation of grouse population dynamics across moors indicates relatively high levels of synchrony between populations on adjacent moors, but that this synchrony declines sharply with increasing inter-moor distance. At distances of greater than 100  km, grouse population time-series exhibit only weakly positive cross-correlation coefficients. Twenty-eight geographical, environmental and other candidate variables were examined to construct a general linear model to explain variation in local synchrony. Grouse moor productivity (average size of shooting bag), distance from the Atlantic coast moving in a north-easterly direction, April and June temperatures, and June rainfall significantly increased the explanatory power of this model. An understanding of the processes underlying synchrony in red grouse population dynamics is a prerequisite to anticipating the effects of large-scale environmental change on regional patterns of grouse distribution and abundance.     

Determinants of the distribution of juvenile fish in the littoral area of a shallow lake

1. Research has often focused on the pelagic areas of lakes; the littoral zone has received less attention. The few studies concerning fish distribution in littoral habitats have concentrated on stands of submersed macrophytes, whereas other littoral habitat types have seldom been investigated. 2. This study aimed to predict the occurrence of juvenile fish in several littoral habitats of a shallow lake as a function of food availability, complexity of habitat structure, water depth and substrate. Habitats comprising reed, woody structures, and two open water areas differing in depth were sampled for fish and invertebrate biomasses on two shores, over 6 months and during both daylight and at night. 3. The juvenile fish community consisted almost exclusively of 0+ and 1+ roach and perch. There was a strong diel component in habitat use, with a predominant occurrence of fish in complex habitats (mainly woody structures) during the day, and a partial migration towards the open habitats at night, more strongly expressed in roach than in perch. 4. The diet of all fish groups was relatively constant over the seasonal cycle, and was independent of habitat. There was a higher degree of planktivory in roach than in perch, but both species fed on benthic macroinvertebrates to a substantial extent. 5. According to a logistic regression model, the biomass of potential food organisms in the different habitats had little predictive effect on the spatial distribution of the fish, whereas the structural complexity of the habitats combined with the diel cycle explained about 28% of the occurrence patterns in 0+ and 1+ perch and 1+ roach.     

Minimal models of top-down control of phytoplankton

1. A set of models describing the dynamics of top-down control of phytoplankton by Daphnia in lakes is reviewed. The basis of these models is a simple and well-known model that has been used, among other things, to demonstrate the paradox of enrichment.
2. We discuss minimal extensions that allow this model to mimic the effects of spatial heterogeneity, planktivory, seasonality and inedible algae.
3. These models generate hypotheses about mechanisms that may cause patterns observed in the field such as:

4. We discuss the way in which such very simple models may contribute to the building of theories about plankton dynamics in the field, and the caveats of interpreting wrongly the message from models.     

The influence of macrophyte beds on plankton communities and their export from fluvial lakes in the St Lawrence River

1. To determine the influence of macrophyte beds on plankton abundance within fluvial lakes of the St Lawrence River, planktonic components (macrozooplankton, heterotrophic bacteria, and phytoplankton as chlorophyll- a [Chl- a ]) were sampled in Lake St Francis and Lake St Pierre during summer 1998. We tested the hypothesis that the abundance of planktonic components was higher within macrophyte beds in comparison to the more rapidly flushed open water areas of the fluvial lakes.
2. The large cross channel variation in zooplankton biomass was indeed correlated with the presence of dense beds of submerged macrophytes. Total macrozooplankton biomass was nine-fold greater within the beds (mean=180 μg L−1 dry mass) than in either the open water or areas with only sparse vegetation (mean=20 μg L−1 dry mass).
3. Chl- a and heterotrophic bacterial abundance were also higher in the beds, but only slightly so. There was no difference in total phosphorus or dissolved organic carbon concentrations between areas of dense vegetation, sparse vegetation or open water.
4. Macrophyte beds on the margins of the fluvial lakes allow the development of high planktonic abundance relative to the fast flowing central channel. Macrozooplankton biomass was much higher at the outflows of the lakes (∼50 μg L−1 dry mass) in comparison to the inflows (<20 μg L−1 dry mass). The increase is due to the transfer of organisms from submerged macrophyte beds into the central channel in the downstream quarter of the two lakes where the marginal littoral waters enter central channel waters.
5. Along rivers, shallow fluvial lakes appear to act as sources of plankton which is exported downstream during years of extensive littoral macrophyte development.     

Nocturnal drift of mayfly nymphs as a post-contact antipredator mechanism

1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom-feeding, nocturnal predators, has rarely been tested.
2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout ( Salmo trutta , a drift-feeding fish), one with alpine bullhead ( Cottus poecilopus , a benthic fish), one with both species, and two fishless streams.
3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non-dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish.
4. In laboratory experiments, bullheads were night-active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre-contact response) and benthic fish (a post-contact response). In streams with bottom-feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.     

Assessing riverscape-scale variation in fish life history using banded sculpin (<Emphasis Type="Italic">Cottus carolinae</Emphasis>)

Advancing the field of fish ecology requires a shift in focus from describing patterns in species occurrences to understanding the mechanisms that regulate distributions and abundances across broad scales. For stream fish ecology, this includes understanding environmental mechanisms that regulate stream fish demographic properties at the scale of stream networks or riverscapes. Despite the fact that Banded Sculpin Cottus carolinae occupy a diversity of habitats and stream sizes across the southeastern United States, relatively little is known about the demography of this species. We assessed annual demographic properties (reproduction, growth, and survival) of C. carolinae collected monthly from four sites distributed longitudinally along the Roaring River riverscape in Tennessee to simultaneously describe life history attributes of the species and address riverscape-scale variation in population dynamics. Cottus carolinae lived for a maximum of four years, local populations were dominated by age-0 and age-1 individuals, reproduction began after one year, spawning occurred during December and January, and mean ova number was 398. A life history tradeoff between growth (robustness) and survival was evident at one site where water temperature and flow were least variable, otherwise life history attributes were consistent across the riverscape despite longitudinal changes in abiotic variables. Our work illustrates the potential for muted population responses to a strong hydrologic gradient in stream size and highlights the stability inherent with fish life history adaptations to natural environmental regimes across broad spatial scales.