Temporal and geographical variation in lake trophic status in the English Lake District: evidence from (sub)fossil diatoms and aquatic macrophytes

1. A sediment core (representing 250–300 years) was taken from each of three lakes of conservation interest and contrasting trophic status in the English Lake District: Wastwater, Bassenthwaite Lake and Esthwaite Water. Lithostratigraphic analyses, radiometric dating and analysis of fossil diatoms were carried out.
2. Transfer functions, based on the diatoms, were used to reconstruct total phosphorus (TP) and, thus, eutrophication at the study lakes. In Wastwater, changes in lake pH were also reconstructed.
3. The lakes were also classified according to their present macrophyte flora, the latter being compared with previous records.
4. The fossil diatoms of Wastwater were continuously dominated by taxa typical of oligotrophic, circumneutral waters, indicating that the lake has not been enriched or acidified in the last 250 years. The aquatic macrophyte flora has probably remained unchanged since before the Industrial Revolution.
5. The diatom assemblages of both Bassenthwaite Lake and Esthwaite Water began to change in the mid-1800s. Further change occurred from the 1960s, at the onset of a recent period of eutrophication. These two lakes have experienced continued nutrient enrichment throughout the 1970s, 80s and 90s, largely associated with increasing phosphorus inputs from sewage effluent. There is no evidence of any recovery in response to recent reductions in external nutrient loads.
6. Only in Esthwaite Water has the change in aquatic macrophytes been pronounced.
7. Palaeolimnological reconstruction is useful in determining background conditions and natural variation in lake ecosystems.     

Effects of aquatic plant management on stream metabolism and oxygen balance in streams

1. In unshaded, nutrient-rich streams, prolific growth of stream macrophytes often results in flows that over-top the banks and in high primary production and respiration that may result in extreme diel variations in dissolved oxygen. Consequently, water protection authorities commonly remove macrophytes periodically.
2. We investigated the effect of plant removal on stream metabolism and oxygen balance in two Swiss streams with a high macrophyte biomass. We monitored the concentration of dissolved oxygen before and after macrophytes were removed by cutting and dredging, and calculated rates of gross primary production and ecosystem respiration by means of diel oxygen curves.
3. The removal of plants, which had reached a dry biomass of 320–420 g m−2 immediately before plant removal, had a different impact on stream metabolism in the two streams. In the first (plants removed in May), neither primary production nor ecosystem respiration were significantly affected. In the second (plants removed in late July), gross primary production and ecosystem respiration were reduced by about 70%. In this latter stream gross primary production increased in the first 2 weeks after plant removal but never recovered to pre-disturbance levels.
4. The removal of plants coincided with only a moderate increase in nocturnal oxygen concentration (+1 mg L−1). This, and the rapid partial recovery of stream metabolism in the second stream, suggests that an increase in the oxygen concentration after plant cutting is transient in unshaded, nutrient-rich streams.     

Assessing the responses of aquatic macrophytes to the application of a lanthanum modified bentonite clay,at Loch Flemington,Scotland, UK

Loch Flemington is a shallow lake of international conservation and scientific importance. In recent decades, its status has declined as a result of eutrophication and the establishment of non-native invasive aquatic macrophytes. As previous research had identified the lake bed sediments as an important source of phosphorus (P), the P-capping material Phoslock^® was applied to improve water quality. This article documents the responses of the aquatic macrophyte community by comparing data collected between 1988 and 2011. Summer water-column total P concentrations decreased significantly and water clarity increased following treatment. Aquatic plant colonisation depth increased and plant coverage of the lake bed extended. However, the submerged vegetation remained dominated by the non-native Elodea canadensis Michx. Aquatic macrophyte community metrics indicated no significant change in trophic status. Species richness and the number of ‘natural’ eutrophic characteristic species remained broadly similar with no records of rare species of conservation interest. Loch Flemington is still classified as being in ‘unfavourable no change’ condition based on its aquatic macrophytes despite the water quality improvements. The implications of these results are discussed in relation to the future management of Loch Flemington and in the wider context of trying to improve our understanding of lake restoration processes.     

Freshwater and wetland plant communities of Loch Lomond

Information on the aquatic and wetland vegetation of Loch Lomond, Scotland, is summarised for the period 1957–90. Aquatic macrophyte growth is estimated to occupy about 1% of the loch's total surface area, being limited to the 0–10 m euphotic zone, and probably excluded from much of this area by wave disturbance and unsuitable substrates. Aquatic vegetation is however abundant in sheltered bays and less-exposed shorelines, particularly in the South Basin of the loch. Although Littorella uniflora (L.) Aschers is the commonest, ubiquitous, submerged plant of the loch, three separate euhydrophyte communities have been identified within the loch. One of these is characterised by abundant stands of Elodea canadensis Michx., which appears to have invaded Loch Lomond some time between 1967–88, and has rapidly spread throughout the loch.     

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.     

Adaptations of fish species to oxygen depletion in a central Amazonian floodplain lake

Pronounced seasonal and daily oxygen concentration changes are characteristic for Amazonian floodplain lakes. Studies on the fish fauna of the Lago Camaleão, Solimões River, Amazonas, Brazil, showed several fish species which are able to survive prolonged periods of heavy hypoxia. Twenty species belonging to eight families were observed in the laboratory in order to determine their respiratory adaptations to hypoxic conditions and oxygen concentrations at which the fish present respiratory adaptations. Finally, the fish species were distributed throughout the habitats of Lake Camaleão according to their adaptation responses. Ten fish species used the surface water for aquatic surface respiration, four species used atmospheric oxygen for aerial respiration, four species used oxygen supplied by the exudation of the roots of floating macrophytes and two exhibited a high tolerance to hypoxic conditions, and well-developed physiological biochemical mechanisms. The fish fauna is well adapted to low oxygen concentrations. The large variety of morpho-anatomical adaptations associated with biochemical and physiological mechanisms to tolerate hypoxic and anoxic conditions enable the 20 fish species to exploit several habitats of Lago Camaleão, such as floating aquatic macrophyte meadows, open water and near the shoreline.     

Release and bioavailability of dissolved organic matter from floodplain litter: influence of origin and oxygen levels

1. We determined the rate of release and microbial uptake of dissolved organic carbon (DOC) leached from three components (leaves, bark and twigs) of river red gum ( Eucalyptus camaldulensis ) forest litter originating from different parts of a floodplain and under different oxygen levels.
2. Preliminary experiments showed that substantially more DOC was released from leaves than from bark or twigs; there was relatively little DOC release from coarse particulate matter or soil.
3. Both the amount of DOC released from each litter component and the amount metabolized by the microbial community were independent of position on the flood-plain or amount of oxygen available to microbes.
4. Although the bioavailability of DOC was independent of oxygen concentration, the microbial utilization of DOC under aerobic and anaerobic conditions differed. Under aerobic conditions, leaves were colonized by fungi, while bacteria were dominant under anoxic conditions.
5. Phospholipid fatty acid profiles of the microbial communities growing on leaf extracts showed that different microbial communities developed in each oxygen concentration treatment suggesting that, irrespective of flood conditions, a microbial community will develop to utilize a significant proportion of the DOC leached from litter.     

Factors influencing macroinvertebrate colonization of seasonal wetlands: responses to emergent plant cover

1. We conducted field experiments to examine factors influencing macroinvertebrate colonization of seasonally flooded marshes. Few macroinvertebrate species were found aestivating in soils within non-flooded wetlands indicating that most taxa colonize these marshes from other flooded habitats.
2. We manipulated amounts of salt grass ( Distichlis spicata ) to examine how emergent plant cover affects aerial colonization by macroinvertebrates. Areas mowed 3 weeks before flooding had low plant cover, areas mowed 5 and 9 weeks before flooding had medium and high plant cover, respectively, and non-mowed control areas had the most plant cover. Macroinvertebrate numbers and biomass were generally higher in mowed treatment areas than in control areas, but overall diversity was generally higher in high plant cover and control areas than in low plant cover areas.
3. Mosquitoes (Culicidae), brine flies (Ephydridae) and hover flies (Syrphidae) were positively correlated with amount of plant cover, and waterboatmen (Corixidae), midges (Chironomidae) and water scavenger beetles (Hydrophilidae) were negatively correlated with plant cover. Species assemblages changed seasonally among treatment areas because these taxa colonize wetlands at different times in the year.
4. These results demonstrate that invertebrate communities may be different within plant stands with heterogeneous amounts of emergent cover, and management practices that alter the structure of wetland vegetation can influence macroinvertebrate communities colonizing seasonal marshes.     

The impact of the crayfish Orconectes virilis on aquatic macrophytes

SUMMARY. 1. The impact of crayfish on the biomass, density and shoot morphology of four submersed plant species was examined under semi-natural conditions. Male or female crayfish ( Orconectes virilis ) were held for 5 weeks at biomasses of 0, 5, 10 or 18 g m⁻² (live weight) in twelve plastic pools (4.67 m², surface area) containing Potamogeton richardsonii, Myriophyllum exalbescens, Nuphar variegatum and Sparganium eurycarpum .
2. Crayfish significantly affected biomass, density and/or shoot morphology of all four macrophyte species. Differences in the effect of crayfish on macrophyte growth were related to plant species, crayfish sex and activity, and the abundance of alternative foods.
3. The effect of female crayfish on macrophyte growth was generally stimulatory. Myriophyllum and Potamogeton biomass, Potamogeton density and Myriophyllum length increased in the presence of female crayfish, possibly due to the reduction in herbivorous snails as a result of crayfish predation. In contrast, plant growth decreased in the presence of male crayfish: Myriophyllum, Nuphar and Potamogeton biomass, Myriophyllum and Sparganium density, and Sparganium and Poiamogeton length were reduced at male crayfish biomasses between 5 and 18 g m⁻².
4. These results indicate that even relatively low densities of crayfish can greatly affect the growth of submersed aquatic plants. Because of their ability to modify aquatic macrophyte, macroinvertebrate and, ultimately, fish communities, the introduction of crayfish into lakes where they do not occur could have a major effect on the structure and composition of the littoral zone.     

Effects of an artificial protection structure on the sandy shore macrofaunal community: the special case of Lido di Dante (Northern Adriatic Sea)

Biomanipulation of eutropicated peaty lakes has rarely been successful; clear water with dense macrophyte stands fails to develop in most cases. It was unclear whether (1) high turbidity due to resuspension by benthivorous fish or wind is the major cause of low macrophyte density or whether (2) the establishment of submerged macrophyte stands is prevented by a lack of propagules, low cohesive strength of the lake sediment, high concentrations of phytotoxics, grazing by waterfowl and/or shading by periphyton growth. These hypotheses were tested in an experiment in a shallow peat lake in the Netherlands (Terra Nova). Removal of fish from a 0.5 ha experimental site resulted in clear water and the development of a dense (90% coverage) and species-rich (10 species) submerged vegetation. At a fish-stocked site and a control site the water remained turbid and dense macrophyte stands did not develop. The establishment of submerged macrophytes appeared not to be limited by a lack of propagules. Introduced plants grew poorly in turbid water, but very well in clear water. Exclosures showed that bird grazing reduced the plant biomass. In clear water grazing seemed to enhance the vegetation diversity. Periphyton development did not prevent plant growth in clear water. After the experiment, the fish stock was greatly reduced in the whole lake (85 ha), to test if (3) in a large lake, submerged macrophyte stands will not develop after biomanipulation. In the first season after fish reduction, transparency increased and species-rich submerged macrophyte stands developed, covering 60% of the shallow parts of the lake. Most of the species known to have occurred in the past re-established. The results indicate that high turbidity caused by benthivorous fish in combination with bird grazing were the major causes of the absence of submerged macrophyte stands in this lake. Abiotic conditions after the clearing of the lake were suitable for the growth of macrophytes. We infer that the restoration potential of submerged macrophyte stands in eutrophicated peaty lakes can be high, and results can be obtained quickly.     

Differential influence of a monotypic and diverse native aquatic plant bed on a macroinvertebrate assemblage; an experimental implication of exotic plant induced habitat

Aquatic plants mediate ecological processes in aquatic habitats, specifically predator–prey (bluegill sunfish (Lepomis macrochirus Rafinesque)-macroinvertebrate) interactions. Macroinvertebrate colonization is directly and indirectly influenced by substrate heterogeneity, interstitial space, and surface complexity. Exotic invasive plant species, such as Hydrilla verticillata L.F. Royle, may alter the available structure in aquatic habitat by creating a shift to a homogeneous habitat, thus affecting the macroinvertebrate community. Since macroinvertebrates provide a food base for young phytophilic fishes, changes in their density and abundance may alter food webs. We investigated the hypothesis that macroinvertebrate community structure is influenced by differences in habitat heterogeneity by measuring difference between a heterogeneous native aquatic plant bed, homogenous hydrilla plant bed, and habitat with no plants. Studies were conducted in the field (pond) and the experimental treatments were: (1) no plants, (2) monotypic bed of hydrilla, and (3) diverse native plants. Aquatic plants, regardless of species, supported greater macroinvertebrate abundance, richness, and biomass. Macroinvertebrate abundance, richness, and biomass in a hydrilla-dominated habitat did not differ significantly from a diverse plant habitat, except for richness in October. Indicator taxa did differ significantly between respective treatments, suggesting a change in species composition. However, no significant effect of fish predation on macroinvertebrate populations and/or community structure was documented. The data suggest that a shift from a natural mosaic of vegetated habitat to a highly complex monotypic habitat (e.g., exotic hydrilla) may reduce spatial heterogeneity important to structuring a macroinvertebrate assemblage. Handling editor: S. M. Thomaz     

The relationship of egg size and incubation temperature to embryonic development time in univoltine and multivoltine aquatic insects

1. We used published data to investigate the combined influence of egg size and incubation temperature on embryonic development time for a broad assortment of aquatic insects at four different incubation temperatures (10, 15, 20 and 25 °C).
2. Embryonic development time (EDT) was positively correlated with egg size at each of the four temperatures, but with different relationships for univoltine and multivoltine aquatic insects. The relationships of embryonic development time to egg size expressed in degree-days did not significantly differ in slope ( P >0.50) or intercept ( P >0.05) for either univoltine or multivoltine aquatic insects at each of the four temperatures.
3. The relationship of embryonic development time (degree-days) to egg mass in multivoltine aquatic insects (EDT=885×0.19, P <0.0001, r 2=0.48) is similar in slope and intercept to that for other oviparous animals (i.e., zooplankton, fish, amphibians and reptiles), and to the relationship of embryonic development time to neonate mass in mammals. Univoltine species on average require 3–5 times longer to develop (EDT=14190×0.29, P <0.001, r 2=0.29) than most other animals of equivalent egg mass, but the relationship of embryonic development time to egg mass is similar in slope to that of most other animals. Together, these relationships provide a basis for evaluating differences in embryonic development time among aquatic insects.     

Large-scale synchrony and inter-annual variability in roach recruitment in the Rhône River: the relative role of climatic factors and density-dependent processes

1. We report patterns of temporal variation in the recruitment of roach ( Rutilus rutilus ). The data consist of the annual abundance of the first 2 year-classes, 0+ and 1+ fish, at four sites in the Rhône River (France) between 1987 and 1997. Over this 11-year period both 0+ and 1+ fish fluctuated strongly.
2. Cross-correlation indicated high spatial synchrony in 0+ dynamics, although correlations among sites in 1+ dynamics were weaker. No clear pattern was apparent in the relationship between the level of synchrony and distance between pairs of sites.
3. The spatial synchrony in 0+ fish could be attributable to large-scale variations in weather, influencing water temperature. Total body length of 0+ roach was correlated with water temperature (expressed in degree-days over 12 °C), and water temperature was the main factor explaining inter-annual variation in 0+ cohort size. Monthly variation in abiotic factors (measured by standard deviation in water temperature and discharge) did not influence 0+ fluctuations. Correlations with June water temperature suggest that year-class strength was mainly determined by abiotic factors during the first few months of life.
4. The absence of spatial synchrony in 1+ fluctuations suggests little correlation between survival and abiotic conditions during the first year of life, other factors influencing survival.
5. Survival in the first year was density-dependent. Intraspecific competition within the 0+ cohort could thus influence the fluctuations in recruitment to older age-classes.
6. The implications of age- or stage-dependent synchrony in temporal variation for species with complex life histories are discussed. Studying spatial synchrony for the different life history stages could enhance our understanding of the population dynamics of spatially structured species.     

湖泊底质与水生植物相互作用综述

简要阐述了湖泊生态系统中底质和水生植物的概念及重要性,综述了底质理化性质对水生植物生长的影响,以及水生植物对底质营养盐的释放和底质再悬浮的作用。通过大量的研究综述回顾论述了不同的湖泊底质类型在一定程度上决定了水生植物的生长状态,适合的底质营养盐范围能促进水生植物生长,不同水生植物对底质营养盐的耐受性有差异。水生植物能促进底质沉降并减少再悬浮,水生植物的存在对沉积物中磷的活性有显著的影响。污染底泥的修复能为水生植物的立地与生长提供了良好的底质条件,有利于富营养化湖泊水生植被的恢复与重建。     

Global diversity of aquatic macrophytes in freshwater

Aquatic macrophytes are aquatic photosynthetic organisms, large enough to see with the naked eye, that actively grow permanently or periodically submerged below, floating on, or growing up through the water surface. Aquatic macrophytes are represented in seven plant divisions: Cyanobacteria, Chlorophyta, Rhodophyta, Xanthophyta, Bryophyta, Pteridophyta and Spermatophyta. Species composition and distribution of aquatic macrophytes in the more primitive divisions are less well known than for the vascular macrophytes (Pteridophyta and Spermatophyta), which are represented by 33 orders and 88 families with about 2,614 species in c. 412 genera. These c. 2,614 aquatic species of Pteridophyta and Spermatophyta evolved from land plants and represent only a small fraction (∼1%) of the total number of vascular plants. Our analysis of the numbers and distribution of vascular macrophytes showed that whilst many species have broad ranges, species diversity is highest in the Neotropics, intermediate in the Oriental, Nearctic and Afrotropics, lower in the Palearctic and Australasia, lower again in the Pacific Oceanic Islands, and lowest in the Antarctic region. About 39% of the c. 412 genera containing aquatic vascular macrophytes are endemic to a single biogeographic region, with 61–64% of all aquatic vascular plant species found in the Afrotropics and Neotropics being endemic to those regions. Aquatic macrophytes play an important role in the structure and function of aquatic ecosystems and certain macrophyte species (e.g., rice) are cultivated for human consumption, yet several of the worst invasive weeds in the world are aquatic plants. Many of the threats to fresh waters (e.g., climate change, eutrophication) will result in reduced macrophyte diversity and will, in turn, threaten the faunal diversity of aquatic ecosystems and favour the establishment of exotic species, at the expense of native species. Guest editors: E. V. Balian, C. Lévêque, H. Segers & K. Martens Freshwater Animal Diversity Assessment     

Drag coefficients of stream bryophytes: experimental determinations and ecological significance

1. Drag coefficients ( C D) of bryophyte-covered rocks were measured to see whether these differed between species. Replicate rocks, each supporting one of six bryophyte taxa, were attached to a base plate mounted on Teflon bearings in a flow tank. The drag force exerted on rocks with and without bryophyte material was measured by a strain gauge at different water velocities.
2. The difference in C D between rocks with and without bryophyte material was calculated for each plant, and expressed as a percentage change ( δC D). This varied significantly from 0 in three of the six taxa. The cushion-shaped moss Bryum blandum increased the C D by around 10%. The moss Blindia lewinskyae and liverwort Cryptochila grandiflora decreased the C D by around 40 and 30%, respectively, presumably reflecting their streamlined growth.
3. Drag characteristics of aquatic bryophytes may help explain differences in their resistance to fast flows. Furthermore, we suggest that some aquatic bryophytes can increase substrate stability by streamlining rocks, rendering them less prone to movement.     

Dwelling at the oxycline: does increased stratification provide a predation refugium for the Lake Victoria sardine Rastrineobola argentea?

1. We tested the hypothesis that increased hypoxia in Lake Victoria provides a refugium for the cyprinid dagaa ( Rastrineobola argentea ) against hypoxia-sensitive Nile perch ( Lates niloticus ).
2. Hypoxia in the main habitat of dagaa was rare during 1979–80, but lasted 3–5 months in 1987 and 1988, during which time adult dagaa spent the day just above the oxycline instead of near the bottom.
3. Dissolved oxygen concentrations in the layer just above the oxycline were not critical for Nile perch, but a mass kill of this species following a sudden upwelling of hypoxic water suggested that oxycline-dwelling is risky.
4. Our data suggest no difference between dagaa and Nile perch in the level at which dissolved oxygen starts to limit their vertical distribution in the water column, but dagaa seems to be even more sensitive to extreme hypoxia (<1–2 mg O2 L−1) than Nile perch.
5. We argue that oxycline-dwelling dagaa are not seeking a predation refugium but that they are limited by low oxygen levels in reaching their feeding areas near the bottom.     

Effects of land use on aquatic macrophyte diversity and water quality of ponds

1. Aquatic macrophyte diversity and water quality of 55 ponds in western Japan were related to land use and morphometric variables to identify the environmental factors that sustain biodiversity and the spatial extent at which these factors operate. 2. The relevant spatial extent for floating‐leaved macrophyte richness (500 m from pond edge) was larger than that for submerged macrophyte occurrence (10, 75 and 100 m), whereas emergent macrophyte richness was best explained at much larger extents (1000 m). Total macrophyte richness was explained at the extent of 500, 750 and 1000 m. The extents relevant for explaining the physicochemical condition of pond water (100 and 250 m) were similar to those for submerged and floating‐leaved macrophytes, suggesting that these two growth forms are more sensitive to water quality compared to emergent macrophytes. 3. Diversity of all three growth forms and that of total macrophytes collectively were inversely related to turbidity and nutrient concentration; among the three growth forms, submerged macrophytes were most affected by water quality. 4. Negative relationships were found between the proportion of urban area and the diversity of the three growth forms and that of total macrophytes and water quality. Species richness of emergent, floating‐leaved and total macrophytes decreased with depth and increased with surface area up to about 5000 m², above which it declined. 5. Urbanisation and enlargement of ponds were the two main factors that decreased aquatic macrophyte diversity in irrigation ponds. To alleviate the adverse effects of urban areas on aquatic macrophyte diversity, our results suggest that management efforts should focus on the creation of buffer zones within the relevant spatial extent from the pond edge.     

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.     

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.