Ecosystem development in different types of littoral enclosures

Macrophyte growth was studied in two enclosure types (gauze and polythene) in a homogeneousPotamogeton pectinatus bed in Lake Veluwe (The Netherlands). The gauze was expected to allow for sufficient exchange with the lake to maintain similar seston densities, the polythene was expected to exclude fish activity and most water exchange. Polythene enclosures held higher totalP. pectinatus biomass (ash-free dry weight, AFDW) than the lake, gauze enclosures were intermediate. The enclosures had a higher abundance of other macrophyte species (Chara sp.,Potamogeton pusillus) than the lake. Seston ash content was not but seston AFDW, periphyton ash content and AFDW were lower in polythene than in gauze enclosures. The difference in plant biomass between gauze and polythene may be attributed to a difference in periphyton density and in seston AFDW due to zooplankton grazing (Rotatoria andDaphnia densities were higher in polythene enclosures). Since seston and periphyton AFDW and ash content were similar in lake and gauze enclosures, the intermediate macrophyte biomass in the gauze enclosures may be explained by reduced wave action and mechanical stress. Alternatively, phytoplankton inhibition by allelopathic excretions from the macrophytes may have caused the high macrophyte biomass in the polythene, and an absence of sediment-disturbing fish the intermediate biomass in the gauze enclosures. Creation of sheltered areas may favour macrophyte growth through both mechanisms and we conclude that this can be an important tool in littoral biomanipulation.     

Bionomics of invertebrates within an extensive Potamogeton pectinatus bed of a California marsh

Fourteen samples of sago pondweed (Potamogeton pectinatus L.) and associated invertebrates were collected every two weeks over a single season of plant growth in a large monospecific pondweed-bed located in Coyote Hills Marsh (Alameda Co., California, USA), using pull-up samplers that collect plants, epiphytic macroinvertebrates, and microcrustaceans throughout the water column. The macro-invertebrate fauna was dominated by insects, primarily chironomids. Semi-aquatic neustonic taxa, including an aphid and a springtail, were common; this is in contrast with most aquatic plant-invertebrate studies, in which neustonic insects are seldom collected because of sampling bias. Over the entire season, P. pectinatus biomass and the densities of four insect taxa (Anopheles spp. mosquitoes, Hydrellia sp. brineflies, Ademon sp. parasitic wasps, and coenagrionid damselflies) were significantly correlated. These correlations resulted from both similar overall phenologies of the plant and each of the insect taxa, and ecological relationships in which P. pectinatus provides either a specialized habitat or food source. macroinvertebrate numbers were highest in mid-summer, when P. pectinatus forms a dense floating canopy; microcrustaceans were more common during plant senescence in early autumn. Individuals of some taxa may be distributed in proportion to plant biomass; this occurred commonly in damselflies, perhaps as a result of territoriality in these nymphs.     

The Effects of Phragmites Removal on Nutrient Pools in a Freshwater Tidal Marsh Ecosystem

Selected nitrogen and phosphorus pools in two freshwater tidal marsh ecosystems on the lower Connecticut River were measured relative to Phragmites, Typha and mixed native wetland plant cover types. For both the Chapman Pond Preserve and Chester Creek Marsh, significant differences were found between plant cover types in porewater ammonium and phosphate for some months during the 1998 growing season; porewater nitrate was always below the detection limit. At Chapman Pond, no significant differences were detected between Phragmites and Typha for plant tissue N concentrations. The standing stock of nitrogen was higher in Phragmites stands, however, owing to its greater aboveground biomass. No significant difference was found between plant cover types for total soil N at Chapman Pond, but KCl extractable ammonium was higher in the mixed cover type than Phragmites or Typha. The results of this study suggest that Phragmites is affecting nutrient pools in freshwater tidal marshes, a result that should be considered in future management design.     

The distribution of submerged aquatic macrophyte biomass in a eutrophic stream,Badfish Creek: the effect of environment

The distribution of biomass of the macrophyte community in Badfish Creek was examined in three sections (A–C) totaling ten kilometers. Biomass samples were taken in a stratified-random manner, with sediment characteristics, depth, current velocity and incident light measured at each site to correlate individual biomass samples with environmental factors. Total community biomass decreased in the downstream section (C), with the biomass ofElodea canadensis decreasing abruptly below section A. The only environmental factors which were correlated with the decrease in macrophyte community biomass, especially that ofPotamogeton pectinatus, in section C was the increase in substrate heterogeneity and sand substrates which lacked surface gravel. The change in substrate was related to channelization. Considering the stream as a whole, the biomass of the dominant species,Potamogeton pectinatus, was correlated with incident light. Other species present wereCeratophyllum demersum andCladophora glomerata. Analysis of riparian vegetation type indicates that tree cover significantly reduced macrophyte biomass by incident light reduction.     

The effect of turbidity state and microhabitat on macroinvertebrate assemblages: a pilot study of six shallow lakes

Shallow lakes can occur in two alternative stable states, a clear-water state and a turbid state. This is associated with separate assemblages of fish, zooplankton and plants. Little is known about whether macroinvertebrate assemblages differ across both stable states. This study investigated this in a connected set of three turbid and three clear-water shallow lakes. To overcome confounding effects of differences in spatial structure of macrophytes in turbid and clear-water lakes, we sampled three microhabitats that occurred in both alternative stable states: open water, sago pondweed (Potamogeton pectinatus) and reed (Phragmites australis). Univariate analyses indicated no differences in the number of organisms, taxon richness or diversity between turbid and clear-water lakes. Multivariate analysis, however, showed significant differences in the macroinvertebrate community structure of both stable states. Nine taxa explained a significant amount of the variation between both lake types, of which seven preferred the clear-water lakes. The number of organisms and the taxon richness were higher in reed than in the other microhabitats, but diversity and evenness did not differ among the microhabitats. Multivariate analyses could separate all three microhabitats. Eight taxa, mainly detritus feeders and collector–gatherers, explained most of the variation in the data and preferred the reed microhabitat. The effects of stable state (6.8% explained variance) and microhabitat (13.1% explained variance) on the macroinvertebrate assemblages were largely independent from each other (1.5% shared variance). Although macroinvertebrates are not implemented in the initial theory of stable states, our results show clearly different assemblages across both stable states.     

Modeling mosquitofish (Gambusia holbrooki) responses to Genapol OXD-080, a non-ionic surfactant, in rice fields

An ecotechnological approach to control crayfish (Procambarus clarkii) infestation in rice fields of the Lower Mondego River Valley (Central Portugal) has recently been investigated. The application of the biodegradable non-ionic surfactant Genapol OXD-080, a fatty alcohol polyglycol ether, in rice paddies at a given concentration (50 mg/l) has been considered as a non-harmful chemical method to mitigate damage caused by crayfish digging activities to rice crops. Therefore, an important requirement regarding the ecological viability of this approach is that populations of non-target species are not significantly affected. A simple mosquitofish (Gambusia holbrooki) population model, in which the relationships with its main food prey were considered, was developed to assess the potential risk to a non-target key species of contaminating the irrigation channels following surfactant application. The model is based on data concerning mosquitofish life cycle and population dynamics, as well as mosquitofish diet and interactions with its main prey species. Quantitative information regarding the acute and sublethal effects of Genapol OXD-080 on mosquitofish and other non-target organisms was obtained from laboratory experiments. Three concentrations of Genapol OXD-080 (0.75, 1 and 2.5 mg/l) were used to simulate a small amount of contamination in irrigation channels. If contamination occurred, the mosquitofish population would tend to decline dramatically, even when submitted to a very small concentration of Genapol OXD-080 (e.g. 0.75 mg/l, 66.7 times lower than the concentration planned to be used in rice paddies). Thus, Genapol OXD-080 could potentially cause vast damage to local mosquitofish populations, and therefore should not be used without taking all precautions to avoid contaminating important biological reservoirs, such as the rice field irrigation channels.     

Photosynthetic plasticity in Potamogeton pectinatus L. from Argentina: strategies to survive adverse light conditions

Argentine Potamogeton pectinatus L. was grown in The Netherlands under laboratory conditions at four light intensities (50, 100, 150 and 200 µE m^–2 s^–1), and photosynthetic performance was evaluated after about 1, 2 and 3 months of growth. At these moments, chlorophyll-a and -b and tissue N and P content were also determined. During the growing period, plant lengths and number of secondary shoots were measured. In the field in Argentina, photosynthetic performance of P. pectinatus was also measured at different light intensities created by artificial shading at various times during the growing season. Field and laboratory photosynthetic results were in good agreement. P. pectinatus showed a significant plasticityin its photosynthesis, rather than in morphology. A fairly constant maximum photosynthetic rate with reduced light enabled the plants to maintain netproduction rates rather unaffected at low light intensities. Still, it can be predicted that increasing turbidity from 1–2 m^–1 at present to 3 m^–1 could lead to a strongly light-limited growth which should reduce the present weed problem considerably. Such a turbidity increase might be achieved by the introduction of a fairly dense bottom-feeding fish population like Common carp (Cyprinus carpio L.).     

Life history of Gambusia holbrooki (Pisces, Poeciliidae) in the Ebro delta (NE Iberian peninsula)

We studied age, growth and reproduction of an introduced poeciliid, Gambusia holbrooki (Agassiz, 1859), in the Ebro delta marshes. Maximum ages observed were 2⁺ in females and 0⁺ in males. Growth commenced in March and continued until September. Females reached greater sizes, had a higher rate of growth and lived longer than males. The largest sizes found were 63 mm versus 32 mm . Multiple spawning occurred throughout the reproductive cycle (from May to September). The average brood size per female reached its maximum in July in the parental cohort (86 embryos/) and in August in the offspring cohort (29 embryos/). After reproduction the parental cohort decreased drastically. The relationship between fecundity (F) and total length (mm) was calculated for each month of the reproductive period. Sex-ratio deviated significantly from unity.Viviparity, several broods and large larvae provide G. holbrooki with several advantages over oviparous species. The capacity to adjust its life history to a particular environment could explain the life history differences between those in the Ebro delta and other populations.     

Spatial and temporal variation of benthic Cladocera (Crustacea) studied with activity traps in Lake Myvatn, Iceland

Benthic Cladocera were studied with a modified type of an activity trap in Lake Myvatn in 1990–1992. After feasibility experiments, the operation time and the distance of the traps from the benthic substrate were adjusted in order to minimize the effects of diurnal variation and the trapping of planktonic organisms. The trap catches of Eurycercus lamellatus were positively correlated with their abundance at the bottom as estimated by grab sampling (r = 0.910, P < 0.001). The usefulness of the activity trap was demonstrated by: (1) a lake-wide survey of the benthic Cladocera; (2) a study of the seasonal variation in the size distribution, abundance and sex ratio of E. lamellatus; and (3) a study of the seasonal succession of Chydorus sphaericus and Alona affinis. The variation of benthic Cladocera among 21 trap sites distributed on a 1–2-km scale across the lake exceeded the within-site variation. The sampling sites could be divided into five main groups based on cluster analysis. Eurycercus lamellatus was the most common species in the mat of filamentous green algae (Cladophorales). Alonella nana dominated the area of spring water inflow in the north basin and Macrothrix hirsuticornis the area of spring water inflow in the southeastern part of the lake. In other parts of the lake either Chydorus sphaericus or Alona quadrangularis tended to dominate. The size distribution and sex ratio of E. lamellatus was followed at two sites through one summer. In early summer most individuals were females less than 1.45 mm long. Around mid-summer they had grown to 0.69–3.1 mm. By the end of August the size distribution had become bimodal, with a large number of small males and a smaller number of much larger females. The seasonal succession in the abundance of E. lamellatus, A. affinis and C. sphaericus was followed at four sites over two seasons. With some exceptions the abundance of a species followed a similar seasonal trajectory on the different stations in any one year. There was, however, a marked difference between the two years (1991 and 1992), probably related to different temperatures.     

Wave exposure related growth of epiphyton: implications for the distribution of submerged macrophytes in eutrophic lakes

The distribution of submerged macrophytes in eutrophic lakes has been found to be skewed towards sites with intermediate exposure to waves. Low submerged macrophyte biomass at exposed sites has been explained by, for instance, physical damage from waves. The aim of this study was to investigate if lower biomass at sheltered sites compared to sites with intermediate exposure to waves can be caused by competition from epiphyton.Investigations were performed in eutrophic lakes in southern Sweden. Samples of submerged macrophytes and epiphytic algae on the macrophytes were taken along a wave exposure gradient. The amount of epiphyton (AFDW) per macrophyte biomass decreased with increased exposure. Biomass of submerged macrophytes, on the other hand, increased with increased exposure until a relatively abrupt disappearance of submerged vegetation occurred at high exposures. Production of epiphytic algae was monitored on artificial substrates from June to September at a sheltered and an exposed site in three lakes. It was higher at sheltered sites compared with exposed sites.We suggest that epiphytic algae may be an important factor in limiting the distribution of submerged macrophytes at sheltered sites in eutrophic lakes.     

Life history, population dynamics and production of eastern mosquitofish, Gambusia holbrooki (Pisces, Poeciliidae), in rice fields of the lower Mondego River Valley, western Portugal

The introduced population of Gambusia holbrooki from the rice fields of the lower Mondego River Valley, Portugal, was studied for 15 months, relating their life cycle and population dynamics with its production, in order to assess the role of the species in the energy flow and secondary production in this type of agro-ecosystem. Two main annual cohorts (1995 and 1996 cohorts) were identified. The females outnumbered males and the average female/male-ratio was 4. The inspection of ovary developmental stages of this viviparous fish, revealed that the most important reproductive period was between April and August. The first recruits were recorded in June and were present thereafter until October. Males from the parental cohort died before August, whereas parental females could survive until October. Mean adjusted fecundity (number of embryos divided by female standard length) peaked in July 1996 (0.95) and in June 1997 (1.05). Females reached greater sizes, had a higher growth rate and lived longer than males. Annual production was estimated at 3.101 g.m^–2.year^–1 (ash-free dry weight, AFDW), the average biomass at 2.896 g.m^–2 (AFDW), and the P/B ratio was 1.071. A conjugation of life history, population dynamics, production and ecological traits (e.g. fast growth, reduced longevity, viviparity, high productivity, an intermediate position in food chain, and no special habitat requirements for reproduction) clearly show that the populations of G. holbrooki, introduced into rice fields all over the world, may play an important role in the structure and functioning of the biological communities of these important agro-ecosystems.     

The genus Synchaeta (rotifers) in a north-western Mediterranean coastal lagoon (Etang de Thau, France): taxonomical and ecological remarks

During a survey of a north-western Mediterranean coastal lagoon, (the Etang de Thau, and at a nearby marine station, 1994–1998), six species of the genus Synchaetawere identified. The systematics of this illoricate genus is difficult and identification is virtually impossible in preserved material, although some indication may be obtained from an examination of the trophi after treatment with sodium hypochlorite. In this study, taxonomic characters (mastax, body morphology and egg size) were obtained from living material prior to preservation. We add some ecological observations: distribution of species (neither temperature nor salinity were found to be determining factors), co-existence (up to five species can co-occur) and consideration of trophic links (competition with tintinnids, copepod nauplii and cirripedes).     

The influence of invertebrates on the breakdown ofPotamogeton pectinatus L. in a coastal marina (Zandvlei,South Africa)

The influence of invertebrates upon the decomposition ofPotamogeton pectinatus L. in a coastal Marina system was examined over 112 days using litter bags. Invertebrate inclusion bags (2 mm mesh, 5 mm holes) registered a dry mass loss of 1% d^–1, while exclusion litter bags (80 µm mesh) produced a 0.4% mass loss d^–1 (a 2.5 fold difference). Losses of ash and N from inclusion bags were greater than those from exclusion bags (p < 0.05). There was a three fold difference between the two treatments in the time taken for litter to breakdown to half the initial stock: T_1/2 for inclusion bags = 43 d, exclusion bags = 130 d. In both treatments, minerals showed an expected rapid loss, due to leaching, with a subsequent slow increase relative to the dry material remaining. A total of nine invertebrate taxa was recorded from inclusion bags, with a peak biomass of 64 mg g^–1 dry massPotamogeton bag^–1 reached at 64 days after immersion. Grazing amphipods,Melita zeylanica Stebbing andAustrochiltonia subtenuis (Barnard), numerically dominated the litter bag fauna, whileM. zeylanica and nymphs of the zygopteran predatorIschnura senegalensis (Rambur) formed most of the biomass. Scanning Electron Microscopy indicated heavy grazing of micro-organisms by invertebrates, with major qualitative differences occurring 112 days after immersion. Invertebrates significantly accelerated the rate of litter breakdown through their feeding activities, assisting fragmentation and thus contributing to plant losses and also by increasing the surface area for microbial colonisation and attack.     

Macroinvertebrates associated with Chara in a tropical coastal lagoon (Imboassica lagoon,Rio de Janeiro,Brazil)

Imboassica lagoon is an urban coastal lagoon located in the municipality of Macaé (RJ), which has been exposed to a process of artificial eutrophication through the inflow of untreated sewage, as well as artificial openings of the sandbar that separates it from the ocean, provoking drastic modifications in this ecosystem. The sampling for the analysis of the community of macroinvertebrates associated with Charopyceae (Chara angolensis e C. fibrosa) were performed between October 1995 and October 1997, with a total of 9 samplings at two sampling stations: one located in the innermost area of the lagoon and another at the mouth of the main sewage channel, close to the sandbar. Throughout this period, four artificial sandbar-opening events were recorded, resulting in changes in the biomass of the macroalgae and in the densities of the macroinvertebrate populations. Through the analysis of the density of organisms per square meter, and density of organisms per 100 g dry weight of Chara, it was established that the biomass of the macroalgae has a direct influence on the density of the associated macrofauna. The greater development of the periphytic community in the area close to the inflow of sewage allows for the development of a community of macroinvertebrates with high densities, composed mainly of scraping organisms, such as the mollusc Heleobia australis (Hidrobiidae).     

Seasonal dynamics of <Emphasis Type="Italic">Zostera noltii</Emphasis> Hornem. in two Mediterranean lagoons

The density, biomass and shoot morphology of two populations of Zostera noltii were monitored from January 1998 to July 1999 at two shallow Mediterranean lagoons of Biguglia and Urbino, which differ in hydro-morphological conditions and nutrient loading. Monitoring included the principal biological and foliar parameters (shoot density, aboveground and belowground biomass, length, width and number of leaves, LAI and coefficient A: percentage of leaves having lost their apex), the organic matter contents of the sediment and the environmental conditions (salinity, turbidity, temperature, nutrient concentrations and dissolved oxygen levels). The two populations of Z. noltii displayed seasonal changes in density (1600–19600 m²), aboveground biomass (11–153 g. DW. m⁻²), leaf length (33–255 mm), and leaf width (0.9–1.8 mm). Temperature and turbidity were significant environmental factors influencing the temporal changes observed in the Z. noltii meadows studied. Conversely, the belowground biomass, the number of leaves per shoot and the LAI did not undergo any seasonal changes. In the Biguglia lagoon, the functioning dynamics of the Z. noltii seagrass beds are determined by the catchment area and the inputs of nutrients derived from it, whereas in the Urbino lagoon the dynamics of the Z. noltiibeds depend on low levels of water turbidity.     

Impact of mass mortality of a mosquito fish,Gambusia affinison the ecology of a fresh water eutrophic lake (Lake Naini Tal,India)

Between 28th March and 4th April, 2000 a fungal infection killed >80% of the most abundant planktivorous fish, Gambusia affinis in Lake Naini Tal, Uttaranchal, India. In response to this mortality, planktonic communities and some eutrophication-related parameters viz., primary productivity, phosphate–phosphorus, nitrate–nitrogen and transparency of the water, were considerably changed. Total zooplankton number more than doubled, phytoplankton number reduced nearly to half, primary production and phosphate-phosphorus was dramatically reduced, while nitrate–nitrogen and water clarity increased. The phytoplankton decline was caused by increased zooplankton grazing (top-down control) rather than phosphorus deficiency (bottom-up control). After 3 months, Gambusia and planktonic communities and nutrient levels reverted back almost to their pre-mortality state. Thus removal of G. affinis could improve water quality of Lake Naini Tal.     

Ecosystem respiration depends strongly on photosynthesis in a temperate heath

We measured net ecosystem CO₂ flux (F _n) and ecosystem respiration (R _E), and estimated gross ecosystem photosynthesis (P _g) by difference, for two years in a temperate heath ecosystem using a chamber method. The exchange rates of carbon were high and of similar magnitude as for productive forest ecosystems with a net ecosystem carbon gain during the second year of 293 ± 11 g C m⁻² year⁻¹ showing that the carbon sink strength of heather-dominated ecosystems may be considerable when C. vulgaris is in the building phase of its life cycle. The estimated gross ecosystem photosynthesis and ecosystem respiration from October to March was 22% and 30% of annual flux, respectively, suggesting that both cold-season carbon gain and loss were important in the annual carbon cycle of the ecosystem. Model fit of R _E of a classic, first-order exponential equation related to temperature (second year; R ² = 0.65) was improved when the P _g rate was incorporated into the model (second year; R ² = 0.79), suggesting that daytime R _E increased with increasing photosynthesis. Furthermore, the temperature sensitivity of R _E decreased from apparent Q ₁₀ values of 3.3 to 3.9 by the classic equation to a more realistic Q ₁₀ of 2.5 by the modified model. The model introduces R _photo, which describes the part of respiration being tightly coupled to the photosynthetic rate. It makes up 5% of the assimilated carbon dioxide flux at 0°C and 35% at 20°C implying a high sensitivity of respiration to photosynthesis during summer. The simple model provides an easily applied, non-intrusive tool for investigating seasonal trends in the relationship between ecosystem carbon sequestration and respiration.     

Fishes of a subtropical Australian saltmarsh and their predation upon mosquitoes

Synopsis The fishes occurring in a subtropical Australian saltmarsh were studied from January to December 1984 to determine the seasonal abundance and feeding of the most common species. Wide variations in pool water levels and salinity were recorded. Eight species of fish were collected, four of which are of economic importance to fisheries. Three taxa, Gambusia affinis, Pseudomugil signifer, and gobiids predominated in terms of abundance and occurrence. The abundance of fishes fluctuated markedly on a seasonal basis with few fish being taken in winter and spring (June–November) when pool water levels were low. Diets of the most common species were similar with terrestrial foods forming an important component of all diets. Feeding activity was highest in summer (December to February) when mosquito larvae, copepods, and larval coleopterans formed a large proportion of the fishes' diets. Results from the present study indicate that fish important to commercial fisheries rarely utilise the upper littoral saltmarsh habitat and that resident fishes are unlikely to effectively control mosquito larvae in saltmarsh.     

Space and time distribution of zooplankton in a Mediterranean lagoon (Etang de Berre)

In the Berre lagoon, a large brackish and swallow area near Marseille, the environmental factors (temperature, salinity, oxygen, suspended particulate matter and chlorophyll) generally display strong space and time variations. The rotifer Brachionus plicatilis and the copepod Acartia tonsa constitute the bulk of the zooplankton population during all the year. Their space and time distributions were studied in 23 stations distributed all over the lagoon, during four seasonal cruises (February, June, October, November), at surface and bottom layers. There is no marked difference in the horizontal and vertical distribution of the two species, (except in November when rotifers were prevailing in surface and copepods at depth) and in their time occurence. When the four series of data are pooled, correlation analysis show that A.tonsa is positively correlated with temperature, salinity and seston and negatively to oxygen and chlorophyll. B. plicatilis is positively correlated with temperature and seston, but also with chlorophyll, while salinity has a negative effect. The specific eggs number of both species is chlorophyll dependent. Considering seasonal cruises separately, some differences appear in the sense or the significance of these different correlations. The respective distribution of the two species is only partly dependent on the variation of the environmental factors: most of the variance remains unexplained, as indicated by the result of a stepwise multiple regression analysis using the most significant factors (temperature, salinity and oxygen explain 33 to 42% of the variance in Acartia, while temperature and salinity explain 27 to 28% of the variance in Brachionus). Thus, internal behavioral factors could also play a role in the distribution of organisms, particularly in some cases of aggregations of organisms observed during this study. As the two species occupied the same space habitat most of the year, they are potentially in competition for food. A way to optimize the food utilization could be the time separation of their feeding activity, nocturnal in Acartia and diurnal in Brachionus. Another way could be selective feeding upon food particles depending on their size (Brachionus being able to use finer particles than Acartia) or their quality (Brachionus being more herbivorous than Acartia) as demonstrated in some grazing experiments carried out in parallel.