Diel variation in concentration,assimilation and respiration of dissolved free amino acids in relation to planktonic primary and secondary production in two eutrophic lakes

Concentration of dissolved free amino acids (DFAA) and assimilation of the 5 most abundant DFAA (glutamic acid, serine, glycine, alanine and ornithine) were measured at 3-h intervals over 27 h in two Danish, eutrophic lakes. The carbon flux of the amino acid assimilation was compared with the major routes of carbon flux, including primary production, bacterial production and zooplankton grazing. In Frederiksborg Slotssø, the mean DFAA concentration was 275 nM with distinct peaks (up to 783 nM) 3 h after sunrise. Assimilation rates of the 5 amino acids amounted on the average to 2.03 µg Cl^–1 h^–1, but high values up to 7.41 µg Cl^–1 h^–1 occurred 3 h after sunrise and at midnight. The mean turnover time of the amino acid pools was 3.2 h. In Lake Mossø, the mean DFAA concentration was 592 nM with peak of 1 161 nM at dusk. The assimilation rate averaged 0.44 µg Cl^–1 h^–1, and the mean turnover time of the amino acid pools was 39 h. In Lake Mossø, similar turnover times of glutamic acid and serine were determined from the ¹⁴C-amino acid tracer technique and Michaelis-Menten uptake kinetics, indicating that the tracer technique gave reliable values of the actual assimilation. The average respiration percentages of the assimilated amino acids were 45% in Frederiksborg Slotssø and 51% in Lake Mossø. Extracellular organic carbon (EOC) released from the phytoplankton contributed DFAA to the water. In Lake Mossø, 81% of the ambient EOC pool was <700 daltons and 9.3% of the EOC was DFAA. This corresponded to about 2.4% of the DFAA pool. Bacterial productivity, determined by means of frequency of dividing cells and ³⁵S-SO₄ dark uptake techniques gave similar results and constituted 4.5 and 3.7 µg Cl^–1 h^–1 in Frederiksborg Slotssø and Lake Mossø, respectively. The bacterial productivity suggested that DFAA were essential substrates to the bacteria, especially in Frederiksborg Slotssø. The zooplankton biomass in Frederiksborg Slotssø was six times larger than that in Lake Mossø, but cladocerans were dominant in both lakes. The zooplankton grazing probably was an important regulatory factor for the bacterial productivity.     

Fluxes of free amino acids in three Danish lakes

SUMMARY. 1. Heterotrophic assimilation rates and concentrations of dissolved free amino acids (DFAA) were followed during diel studies in the eutrophic Lake Mossø, Lake Esrom and Lake øm in spring and summer in 1982. In all three lakes, three to four fold diel variations in concentrations and assimilation rates were measured. These fluctuations appeared to be iindependent of phytoplankton and bacteria production. Pools of DFAA varied from 380 nM (Lake Mossø) to 2430 nM (Lake ørn), with serine, glycine, alanine and ornithine as dominant free amino acids.
2. When similar water samples were incubated in a natural light-dark cycle or in total darkness, different pools of DFAA were measured in light and dark.
3. Decomposition of organic matter or zooplankton activity (rather than e.g. phytoplankton exudates) appear to be responsible for the concentration changes.
4. Observed discrepancies between simultaneous concentration changes and assimilation rates are discussed in relation to the applied tracer procedure and the concentration measurements.
5. Assimilation of DFAA sustained from 6% to 25% of the bacterial carbon requirement, corresponding to 2–12% of the phytoplankton production in the lakes.     

Seasonal changes in the dissolved free amino acid and DOC concentrations in a hypertrophic African reservoir and its inflowing rivers

Dissolved free amino acid (DFAA) concentration and composition and dissolved organic carbon (DOC) concentration were measured over 16 months at three depths in hypertrophic Hartbeespoort Dam, South Africa and in its two perenially inflowing rivers. The range of DFAA concentrations in the reservoir and both rivers were similar with dominant DFAA consisting of serine, glycine, alanine and ornithine in all three systems. The range of DOC concentrations in the rivers was 1.5–11.1 mg l^–1, the major river (Crocodile) having about twice the DOC concentration of the Magalies River. The DFAA/DOC ratios ranged between 0.02–1.1% in the Crocodile River and 0.13–3.7% in the Magalies River. DFAA and DOC concentrations were positively correlated to the Magalies River flow, but for the Crocodile River, which received domestic and industrial effluents, DOC was inversely correlated to flow. The source of DFAA in both rivers was mainly terrestrial, in contrast to the main DOC source in the Crocodile River which was the effluents. The DFAA load of the Crocodile River ranged between 0.22 and 208 kg C d^–1.DOC (5.0–24.8mg l^–1) in Hartbeespoort Dam generally decreased with depth but DFAA (15–4800 nmol l^–1) concentration showed no clear trend. The DFAA/DOC ratios varied between 0.02 and 2.9%. DFAA concentrations were correlated (r = 0.3, n = 30, p = 0.04) with bacterial numbers at 0 and 10 m only while no significant correlations were found with bacterial production, chlorophyll a concentration and phytoplankton primary and EDOC (extracellular DOC) production at any depth. The rate of bacterial utilization of DFAA was low compared with data from other lakes. Diurnal phytoplankton production of DFAA in the euphotic zone of the whole lake was calculated to vary between 268 and 30 780 t C d^–1 indicating autochthonous DFAA sources were dominant to allochthonous DFAA sources. The autochthonous production of DFAA was > 2 × gross bacterial production of the euphotic zone indicating that although DFAA concentrations were frequently < 10 g C l^–1, the rate of DFAA production exceeded bacterial requirements.     

Occurrence and Bacterial Cycling of d Amino Acid Isomers in an Estuarine Environment

Abundance of d isomers of amino acids has been used in studies of organic matter diagenesis to determine the contribution of bacterial biomass to the organic matter, especially in marine sediments. However, fluxes of d amino acids in pelagic waters are poorly known. Here we present seasonal changes (March–September) in concentrations of dominant d amino acids in the pool of dissolved free and combined (hydrolysable) amino acids (DFAA and DCAA) in the shallow Roskilde Fjord, Denmark. The amino acid dynamics are related to pelagic bacterial density and activity and abundance of viruses. d␣isomers made up 3.6 and 7.9% of the DFAA and DCAA (average values), respectively, and had similar seasonal variations in concentrations. In batch cultures (0.7- and 0.2-m filtered water in a 1:9 mixture) microbial activity reduced l+d DCAA concentrations in seven of ten sampling dates, while DCAA were released at the remaining three sampling times. NH₄⁺ balance (uptake or release) in the cultures correlated significantly with variations in concentrations of d-DCAA, but not with the total DCAA pools. Abundance of viruses did not correlate with density or production of bacteria in the fjord, but covaried with mineralization of total C, DCAA and PO₄³⁻ in the batch cultures. The content of d amino acids in bacterial biomass in the cultures varied from 6.7 to 12.5% and correlated with the d isomer concentration in the fjord, except for d-Ala. In an additional six-day batch culture study, DCAA and d-DCAA were assimilated by the bacteria during the initial 36 h, but were released between 36 and 42 h simultaneous with a decline in the bacterial density. Our results demonstrate that peptidoglycan components contribute to natural amino acid pools and are assimilated by bacterial assemblages. This cell wall “cannibalism” ensures an efficient recycling of nutrients within the microbial community. Significant positive correlations between viral abundance and bacterial mineralization of organic matter in the fjord indicated that viral lysis contributed to this nutrient recycling.     

Colloidal and dissolved organic matter in lake water: Carbohydrate and amino acid composition,and ability to support bacterial growth

Bacterial utilization of dissolved organic matter (DOM) was studied in water from a humic and a clearwater oligotrophic lake. Indigenous bacteria were inoculated into either 0.2 m natural filtered lake water, or lake water enriched fivefold with colloidal DOM >100 kD but below 0.2 m. Consumption of DOM was followed from changes in concentrations of total dissolved organic carbon (DOC), dissolved combined and free carbohydrates and amino acids (DCCHO and DFCHO, and DCAA and DFAA, respectively) and by uptake of monosaccharide and amino acid radioisotopes. DCCHO and DCAA made up 8% (humic lake) to 33–44% (clear-water lake) of the natural DOC pools, while DFCHO and DFAA contributed at most 1.7% to the DOC pools. Addition of >100 kD DOM increased the DOC concentrations by 50% (clearwater lake) to 92% (humic lake), but it only resulted in a higher bacterial production (by 63%) in the humic lake. During the incubations 13 to 37% of the DOC was assimilated by the bacteria, at estimated growth efficiencies of 4–8%. Despite the measured reduction of DOC, statistically significant changes of specific organic compounds, especially of DCCHO and DCAA, generally did not occur. Probably the presence of high molecular weight DOC interfered with the applied analytical procedures. Addition of radiotracers indicated, however, that DFAA sustained 17–58% and 29–100% of the bacterial carbon and nitrogen requirements, respectively, and that glucose met 1–3% of the bacterial carbon requirements. Thus, our experiments indicate that radiotracers, rather than measurements of concentration changes, should be used in studies of bacterial utilization of DOC in freshwaters with a high content of humic or high molecular weight organic matter.     

Pelagic food web processes in an oligotrophic lake

Major pelagic carbon pathways, including primary production, release of extracellular products (EOC), bacterial production and zooplankton grazing were measured in oligotrophic Lake Almind (Denmark) and in enclosures (7 m³) subjected to artificial eutrophication. Simultaneous measurements at three days interval of carbon exchange rates and pools allowed the construction of carbon flow scenarios over a nineteen day experimental period.The flow of organic carbon was dominated by phytoplankton EOC release, which amounted from 44 to 58% of the net fixation of inorganic carbon. Gross bacterial production accounted for 33 to 75% of the primary production. The lower values of EOC release (44%) and bacterial production (33%) were found in the enclosures with added nutrients. The release of recently fixed photosynthetic products was the most important source of organic carbon to the bacterioplankton. Uptake of dissolved free amino acids was responsible for 52 to 62% of the gross bacterial production. Thus, amino acids constituted a significant proportion of the EOC. Zooplankton (< 50 µm) grazing on algae and bacteria accounted only for a minor proportion of the particulate production in May. Circumstantial evidence is presented that suggests the chrysophycean alga Dinobryon was the most important bacterial remover.The results clearly demonstrated EOC release and bacterial metabolism to be key processes in pelagic carbon cycling in this oligotrophic lake.     

Vertical and temporal distribution of two copepod species, Cyclops scutifer and Diaptomus pribilofensis, in 24 h arctic daylight

Factors structuring zooplankton communities in areas with 24h of sunlight are not well understood. In stratified temperatelakes with fish, zooplankton generally undergo a diel verticalmigration (DVM) to avoid predation. Zooplankton abundance mayalso be linked to the quality and quantity of food. To explorefactors that determine vertical structure of zooplankton whereDVM does not occur, we obtained time-series data on the verticaldistribution of the two dominate copepods, Cyclops scutiferand Diaptomus pribilofensis, in Toolik Lake, Alaska, a sitewithin the Arctic Long Term Ecological Research area. Thesetwo species compose 80–90% of pelagic mesozooplanktoncommunity density. Temperature plays a large role in the verticaldistribution of these two species. During the summers of 2002and 2003, C. scutifer underwent a seasonal vertical migrationfrom the epilimnion to the hypolimnion as water temperaturesincreased. Diaptomus pribilofensis exhibited a strong preferencefor warmer water and were not present in the water column untilepilimnetic temperatures reached 9°C. During both yearsof this study, C. scutifer and D. pribilofensis segregated verticallyin the water column along thermal gradients when the two speciesoverlapped temporally.     

Contribution of Bacterial Cell Wall Components to DOM in Alkaline,Hypersaline Mono Lake,California

Soda lakes are often characterized by high densities of prokaryotes and high concentrations of dissolved organic carbon. Since bacterial cell walls are less degradable than most other cell constituents, accumulation of cell wall material may occur in these lakes and contribute to the DOM pool, but composition of DOM in soda lakes has rarely been examined. Here we report concentrations of DOM components likely originating from bacterial cell walls, including D amino acids, glucosamine (GluA) and muramic acid (MurA), in depth profiles of stratified, alkaline, hypersaline Mono Lake, CA. Concentrations of cell wall components were related to total pools of dissolved free and combined amino acids (DFAA and DCAA), and bacterial density and production. In the free pool, total DFAA ranged from 50 to 3250 nM and typically increased with depth, while GluA (5 to 140 nM) and MurA (< 0.5 nM and only detected in 2005) fluctuated with depth. In the combined pool, DCAA varied between 5000 and 15000 nM and did not show clear depth-related trends. GluA ranged from 1000 to 5000 nM and tended to increase in the hypolimnion, while MurA varied between 25 and 75 nM. Free D isomers in the DFAA pool either made up < 13% (Asp and Ser) or varied from 10 to 57% (Glu and Ala). In the combined pool, D isomers of Asp, Glu, Ser and Ala made up 24-48% of these DCAA and typically showed minor changes with depth. In 2005, lysozyme activity had highest rates in the surface and correlated negatively with most D isomers among the combined amino acids. Our observations demonstrate that the pool of dissolved combined amino compounds in the lake was about 5-fold higher than in other eutrophic lakes and that a substantial portion of these amino compounds originated from bacterial cell walls.     

Life history characteristics and distribution of Bythotrephes longimanus Leydig (Crustacea,Onychopoda) in the Biesbosch reservoirs

The diel and horizontal distributions of Bythotrephes longimanus Leydig, Leptodora kindti (Focke) and Anomopoda (cladocerans), as well as several life history parameters of each developmental stage of B. longimanus were studied in the non-stratified Biesbosch reservoirs. Anomopoda avoided the surface layer during the day only in the pelagic zone. In the surface layer near the shore, however, they were very abundant. Because densities in the pelagic zone during the day were very low, and high during the night, the population undoubtedly moved from the littoral zone or from the sediments to the pelagic. Accumulation of zooplankton at the littoral site is either the result of random dispersal or wind-induced movements. Bythotrephes' diel distribution pattern is possibly related to that of their prey, the anomopods. Leptodora mostly exhibited a normal diel vertical migration with a small amplitude. Wind most likely influenced the horizontal distribution of the zooplankton. Bythotrephes body length increased with higher temperatures, whereas spine length was constant throughout the year. Bythotrephes can already be fertile in the first developmental stage, indicating that a rapid adaptation to a favourable environment is possible. The number of parthenogenetically produced eggs per ovigerous female was higher at the start of the growing season and constant throughout the rest of the year. Relatively few resting eggs and males were found in autumn.     

Seasonal patterns of total and energy reserve lipids of dominant zooplanktonic crustaceans from a hyper-eutrophic lake

Summary Seasonal patterns of lipid reserves and lipid classes of dominant zooplankton in a hyper-eutrophic lake were examined in relation to algal food resources. Triacylglycerol was the principle lipid energy reserve in all five species examined. During the height of the yearlyAphanizomenon flos-aquae bloom, lipid levels of the principle herbivores (Daphnia pulex andLeptodiaptomus sicilis) and an omnivore (Diacyclops bicuspidatus thomasi), were at their lowest concentration, suggesting that this cyanobacterium is nutritionally inadequate. As the cyanobacterial bloom began to collapse, bacterial numbers increased rapidly. The increase in bacterial numbers coincided with a large increase in areal lipid energy reserves ofDiaphanosoma leuchtenbergianum andChydorus sphaericus. Examination of seasonal patterns in the biomass of different algal species suggested thatRhodomonas minuta andCryptomonas erosa played a key role in nutrition, lipid deposition, and reproduction ofD. pulex andL. sicilis.     

Diel changes in plankton and water chemistry in Wicken brickpit

In a flooded Fenland brickpit 3 metres deep a 24-hr study (supplemented by other observations in the field and in the laboratory) revealed marked changes with depth and time in oxygen concentration, pH, total carbon dioxide, ammonium, oxidised nitrogen, phosphate and silica; as well as changes in the rate of cell-division and the vertical distribution of phytoplankton (Dinobryon and Peridinium), and the feeding and vertical movement of zooplankton (copepods, Bosmina, Ceriodaphnia, Polyarthra and Keratella). Directional trapping of zooplankton revealed relationships between population density and the intensity of locomotory activity (a relationship subsequently supported by laboratory experiments), and between the rate of change of light intensity and the direction of swimming. Significant temporal segregation of the occupancy of a given level by zooplankters implies interspecific competition. Transient peaks in the concentrations of some nutrients near the surface are tentatively attributed to nutrient release by zooplankters that have fed at depth. Small-scale temporal and spatial heterogeneity of the type described here may help to explain the paradox of the plankton.These marked diel changes in water chemistry suggest that conclusions based on the analysis of single water samples should be viewed with caution.[/p]     

Zooplankton community response to reservoir aging

Changes in zooplankton diversity and density in response to reservoir aging in Pawnee Reservoir were investigated. Zooplankton samples collected from April 1992 through April 1993, were compared to a similar study conducted after initial impoundment by Helzer (1971), in 1968–1970. Since this initial study, increases in turbidity and resulting changes in biotic interactions significantly altered the zooplankton community. A significant increase in total zooplankton density and a decrease in species richness were observed between study periods. Density increased from 24.6 to 95.4 individuals L^–1, while the number of taxa declined from fourteen to ten. During this time period, Cyclops vernalis became the dominant zooplankter in the reservoir. The density of this predatory copepod increased significantly, from 0.1 l^–1 in 1968–1970, to 44.3 l^–1 in 1992–1993, which accounted for most of the increase in total zooplankton density. Though a greater spring maximum of another dominant, Bosmina spp. was found during the 1992–1993 study period, the annual density of this cladoceran was not significantly different between study periods. Similar trends for Daphnia ambigua and D. parvula were also observed, as greater spring maxima levels were attained, however overall annual densities were not significantly different. The dominance of C. vernalis (46% of annual density) and Bosmina spp. (33%), indicate that these two zooplankters were tolerant of changes in physical conditions resulting from reservoir aging and biotic interactions that followed in the reservoir during the 22 years between study periods.     

Interactions between freshwater bacteria andAnkistrodesmus braunii in batch and continuous culture

Batch and continuous cultures ofAnkistrodesmus braunii were established in an inorganic medium with growth rate limited by P. In batch culture, inoculation of lake water bacterial isolates ofPseudomonas sp. andFlavobacterium sp. showed that thePseudomonas isolate was capable of more rapid growth on algal exudates of lytic products than was theFlavobacterium isolate. When inoculated singly into a continuous culture (D=0.267 day^–1; P level, 2M), theFlavobacterium isolate initially caused a decrease in the population density of the alga, but then steady states for both organisms were obtained. ThePseudomonas isolate under the same conditions caused a rapid washout of the algal culture, and steady-state conditions were never obtained. When thePseudomonas isolate was added to the two-member, steady-state system ofA. braunii andFlavobacterium, the algal population again washed out of the vessel, followed by theFlavobacterium and then thePseudomonas isolate. A transient increase in the P concentration to 200M in the culture vessel caused the low algal population level to increase, followed by increases in the bacterial isolates when the algal population was high enough to supply the required organic carbon source. The system demonstrated that competition for P between the alga and the bacteria can occur, and the results were dependent on the algal and bacterial relative growth rates. The bacterial growth rates were limited initially by organic substrates produced by the alga, and the different bacterial isolates competed for these substrates.     

Vertical, lateral and longitudinal movement of zooplankton in a large river

1. The spatial distribution and movement patterns of zooplankton in large rivers are little known compared with those in lake environments. We conducted a series of studies in the Ohio River (U.S.A.) during the low flow period to assess diel vertical (DVM), longitudinal and lateral movement of crustacean zooplankton. 2. The dominant large zooplankter, the copepod Eurytemora affinis, showed a consistent vertical migration pattern of daytime ascent and night‐time descent during all sampling periods – the reverse of the most common migratory pattern of zooplankton in lakes. The cladoceran Bosmina migrated in a similar way in two of the three sampling periods. Surveys taken longitudinally in the river showed similar trends for both taxa. 3. During the lateral surveys, E. affinis was significantly more abundant in the shallow littoral zone during the night than in the daytime. The combination of vertical and lateral movement patterns along with the diel distribution of zooplanktivorous fish suggest that these movements are a predator‐avoidance mechanism. 4. Sampling programmes in large rivers should consider that larger zooplankton such as E. affinis may not be randomly distributed in the river channel and behaviours such as diel vertical migration may be just as evident in river habitats as in lakes.     

<Emphasis Type="Italic">Daphnia</Emphasis> versus copepod impact on summer phytoplankton: functional compensation at both trophic levels

Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalina×galeata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 µm³), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.     

Significance of algal extracellular products to bacteria in lakes and in cultures

In simulated diurnal experiments withChlorella pyrenoidosa andPseudomonas fluorescens, bacterial growth was virtually confined to the daylight period and occurred at the expense of glycolate, the predominant extracellular product of the alga. Both glycolate levels and¹⁴C-DOC excretion rates were much lower in mixed algal-bacterial than in axenicChlorella cultures.This close coupling of bacterial growth to algal photosynthesis and extracellular release was also observed in Jack's Lake, but not in Lake Erie. Experimental enrichment with lake water particulates >30m suppressed the daytime growth of bacteria in Jack's Lake, but increased it dramatically in Lake Erie. Daylight doubling times for bacteria in lakewater ranged from 2 to 19 days. In mixed culture withChlorella, Pseudomonas had a doubling time of about 2 hours in the light.     

Cascading effects of the flathead grey mullet Mugil cephalus in freshwater eutrophic microcosmos

Flathead grey mullet (Mugil cephalus L.) stocked in fish ponds were long considered to feed primarily on detritus. However, recent research has found that they obtain much of their food from plankton and that they have a detrimental effect on pond zooplankton and large phytoplankton, whilst enhancing small phytoplankton. It has been suggested that flathead grey mullet may also increase the internal phosphorus loading of the ecosystem, which would also increase phytoplankton density. To test whether zooplankton removal or nutrient release from the sediment is the better explanation for phytoplankton enhancement in the presence of flathead grey mullet, the ecosystems of fish-less tanks, tanks with a 60 m mesh filter and tanks stocked at a fish density of 243 g m^-3 were compared. In the presence of flathead grey mullets, cladocerans, ostracods and chironomid larvae became scarcer than in the control tanks, while there were more small phytoplankton and mud snails. The green algae Cladophora sp. did not occur at all. The presence of a mechanical filter also reduced cladoceran, ostracod and chironomid densities and increased phtyoplankton and mud snail density. However, the situation observed in filter tanks was intermediate between that observed in the fish tanks and the control tanks, due to the lower filtering efficiency of the mechanical filter. The organic matter content of the sediment decreased throughout the experiment in the control and filter tanks, but remained stable in fish tanks. Phosphorus and nitrogen concentrations were not affected by any treatment. These results showed that flathead grey mullet enhanced phytoplankton development due to the removal of large cladocerans and not as a consequence of nutrient release. Furthermore, the flathead grey mullet strongly modified the benthic community, probably due to direct predation.     

Predator-dependent diel migration by <Emphasis Type="Italic">Halocaridina rubra</Emphasis> shrimp (Malacostraca: Atyidae) in Hawaiian anchialine pools

Diel migration is a common predator avoidance mechanism commonly found in temperate water bodies and increasingly in tropical systems. Previous research with only single day and night samples suggested that the endemic shrimp, Halocaridina rubra, may exhibit diel migration in Hawaiian anchialine pools to avoid predation by introduced mosquito fish, Gambusia affinis, and perhaps reverse migration to avoid the predatory invasive Tahitian prawn, Macrobrachium lar. To examine this phenomenon in greater detail, we conducted a diel study of H. rubra relative abundance and size at 2-h intervals in three anchialine pools that varied in predation regime on the Kona-Kohala Coast of Hawai‘i Island. We found two distinct patterns of diel migration. In two pools dominated by visually feeding G. affinis, the abundance of H. rubra present on the pool bottom or swimming in the water column was very low during the day, increased markedly at sunset and remained high until dawn. In contrast, in a pool dominated by the nocturnal predator M. lar, H. rubra density was significantly lower during the night than during the day (i.e., a pattern opposite to that of shrimp in pools containing fish). In addition, we observed that the mean body size of the shrimp populations varied among pools depending upon predator type and abundance, but did not vary between day and night in any pools. Our results are consistent with the hypothesis that H. rubra diel migratory behavior and size distributions are influenced by predation regime and suggest that diel migration may be a flexible strategy for predator avoidance in tropical pools where it may be a significant adaptive response of endemic species to introduced predators.     

Changes in the moorland pools near Oisterwijk between 1840 and 1975

Summary The moorland pools (vennen) near Oisterwijk are originally oligotrophic, because the soil consists of nutrient poor cover sands. The original vegetation of most pools belongs to theIsoeto-Lobelietum and the consociation ofCarex rostrata andSphagnum.