Marginal Vegetation Research Articles

Habitats of 0+ fry in an English lowland river

The fish fry communities of the upper River Thames and a connected backwater were sampled quantitatively by a fry seine net in July-August when river flows were very low and the natural marginal vegetation was fully developed. At this time of year, the habitats available to them were (1) clumps of channel plants at the water edge of the littoral vegetation and (2) deep erosion banks without plants. A third characteristic habitat in this river were (3) shallow organically-rich sandy-gravel beaches produced by watering cattle. These three habitats were sampled along a 25 km stretch of the main channel in order to estimate species composition, densities and sizes of their fry communities. A comparable study in a flow-through backwater sited on the original river course showed that fast water flows from an open weir-gate influenced water depths, produced steeper inshore gradients and affected the distribution of the marginal vegetation and the fry communities. The most species-rich and abundant fry communities were found in habitats with a plant cover and in the shallow beaches because they offered both a good food resource and protection from predation. Neither of these was available in the deep erosion banks where few 0+ fish occurred. No 0+ fry occupied the plantless sites in the backwater which were subject to high velocities although even a poor plant cover could offer a refuge against flow for a few species. Roach was the predominant species in the abundant fry communities and its relative abundance explained differences in community structure estimated by Simpson's and Shannon Diversity Indices.

North African wetland lakes: characterization of nine sites included in the CASSARINA Project

Exploitation of land and water resources has increased rapidly in North Africa during the 20th century, paralleling regional population growth. As part of the CASSARINA Project (see Flower, 2001), the environmental status of nine wetland lakes in Morocco, Tunisia, and Egypt was evaluated. All are conservationally important habitats and several are Ramsar Sites (internationally recognized bird reserves) and several support significant fisheries. All are shallow (<2 m in depth) but vary greatly in area. Where available, documentary information on relevant 20th century changes is given. Survey transects for aquatic vegetation were established and used to provide baseline ecological information on the aquatic plant communities during 1997–1999. Unusually, one site (Tunisian Megene Chitane) supported acidophilous vegetation (some taxa being nationally rare). Aquatic macrophytes declined catastrophically at two sites during the 1990s. Merja Bokka was drained in 1998 and, at Garaet El Ichkeul, fringing Phragmites and Scirpus spp. were lost, mainly as a result of salinity changes. Elsewhere, fringing macrophytes remain (extensively so in the Nile Delta lakes) common, despite major land reclamation and water quality problems, or are degraded by grazing (Merja Zerga). Marginal vegetation during 1997/98 changed markedly at Megene Chitane due to water level lowering. Documentary records indicated that throughout the 20th century, reclamation and hydrologic modifications, mainly for agricultural purposes, affected all nine sites. The loss of lake area by reclamation is substantial for the Nile Delta lakes (Edku, Burullus and Manzala). For the western sites, some data indicate increasing salinity in the most recent decade but the Delta lakes have become generally fresher during the 20th century, as supply of Nile water for irrigation increased. Despite intense human disturbance, many of the remaining CASSARINA sites still support regions of high aquatic diversity. Spatial scale monitoring of the larger sites for seasonal and inter-annual changes in open water area and in aquatic plant abundances is a key requirement for integrated environmental change assessment in the 21st century.

The importance of aquatic margins to invertebrates in English chalk streams

Standardized kick-sweep samples of invertebrates were taken from three habitats - mid-channel gravel, Ranunculus and marginal vegetation - in six lowland chalk streams, on four sampling occasions between May 1996 and January 1997. Several types of marginal vegetation were sampled, including the stems and leaves of trailing terrestrial vegetation and semi-emergent marginal macrophytes. There was little difference in invertebrate community composition between the different marginal vegetation types, despite fine-scale differences in habitat architecture. The invertebrate communities in mid-channel gravel, Ranunculus and marginal vegetation were very different from each other over most of the year, particularly in spring and winter. Each habitat was characterised by a suite of invertebrates, reflecting the physical and biological differences between the habitats. Community diversity and equitability were greatest in the physically complex margins and least in the more simply structured Ranunculus. Invertebrate abundance per sample was greatest in Ranunculus. Approximately one half of all taxa in the chalk streams showed no significant preference for any single habitat. Of those taxa that significantly preferred one habitat, many more preferred marginal vegetation compared to either gravel or Ranunculus. Many taxa found throughout the investigation avoided gravel and Ranunculus, but very few taxa avoided marginal vegetation. Rare taxa also occurred more frequently in the margins. Marginal vegetation was used by invertebrates both as a favoured site of oviposition and as a conduit between the aquatic and terrestrial environments. Margins thus acted as a focus for reproduction and recruitment for chalk streams. The relative importance of marginal vegetation for stream invertebrate biodiversity and reproduction is high relative to other mid-channel habitats. More attention should be given to this habitat than has been in the past, particularly in the fields of stream conservation and restoration.

Summer spatial patterns of the fish community in a large, shallow, turbid coastal lake

Several water quality parameters, marginal vegetation, and the fish community in Lake Ellesmere, a large, shallow, turbid lake situated on the east coast of the South Island, New Zealand, were sampled along selected transects in late summer to determine species spatial patterns. Thirteen species offish were caught, with seven of them comprising major populations, although only four of these were adequately sampled by the method used—they included inanga (Galaxias maculatus), common smelt {Retropinna retropinna), shortfinned eel (Anguilla australis), and common bully (Gobiomorphus cotidianus). Inanga were almost exclusively confined to areas on the western side of the lake which has a convoluted shoreline with a variety of small embayments with scattered vegetation along the margins. Common smelt occupied both marginal and offshore areas, but they too were more abundant along the margins particularly on the western side. Both the common bully and shortfinned eel were widespread and abundant in the lake, although their abundance and biomass were higher within the vicinity of the lake outlet, major inflowing tributaries, and other marginal habitats. All three species of flounders of the genus Rhombosolea were poorly represented in our samples. However, in commercial catches they were reasonably abundant, with the sand flounder (R. plebeia) and yellowbelly (R. leporina) being more common in offshore than inshore catches, whereas the black flounder (R. retiaria) showed no difference in the catches between areas. Our overall findings are consistent with the hypothesis that bottom‐dwelling species (eels, bullies, flounders) are more widely distributed than free‐swimming forms (inanga, common smelt), which were largely confined to areas sheltered from prevailing winds. The highly turbid nature of the lake, primarily because of re‐suspension of fine bottom sediments by frequent winds, is considered an important factor in the distribution of the fish fauna.

Transient Storage and Gas Transfer in Lowland Stream

Conservative and gaseous tracers are used in conjunction with several models to characterize transport and gas transfer for a small, sluggish lowland stream, the Piako River, in New Zealand. Results obtained with the transient storage model indicate that storage zones make up a significant proportion of the total stream cross section. This result is consistent with the fact that the river is lined with profuse marginal vegetation. However, because of their high density, exchange between these areas and the main channel is minimal. Hence, their net impact is to increase velocity and depth, rather than to trap pollutants. Measurement of gas exchange indicates that, although storage zone models fit tracer profiles better, the effect of transient storage on the estimation of gas transfer rates is negligible for systems like the Piako.

The impacts of dredging and weed cutting on a population of freshwater mussels (Bivalvia: Unionidae)

Regulated lowland rivers generally require management to control macrophytic vegetation and sediment build-up. Such management can have deleterious effects on much of the biota. It has long been a concern that indiscriminant river management has played a part in the world-wide decline in freshwater mussel populations. This study investigated the impact of dredging and weed-cutting on the population size, structure and distribution of four species of unionid mussel: Anodonta anatina, A. cygnea, Unio pictorum and U. tumidus. Dredging removed between 3% ( A. anatina) and 23% ( A. cygnea) of the mussel population. The weed bucket removed a maximum of 3% of any species, but its higher frequency of use results in removing a similar number of mussels to dredging in the long-term. Marked stones placed in the river during dredging suggest that the excavator drags mussels across the river bed. This is supported by the relatively high density of mussels in the channel closest to the bank from which the excavator habitually operates. Tagged mussels moved only small distances following dredging (generally <15 cm after 55 days) and showed no tendency to disperse. The impact of river management on unionid populations can be reduced, while retaining its channel maintenance function, by dredging and weed cutting only within the centre of the channel. Marginal vegetation should be cut to 5 cm above the river bed using weed boats with an annual alternation between banks to preserve refugia of invertebrates and fish.

An investigation of marginal habitat and macrophyte community enhancement on the River Torne, UK

During floodbank raising work as part of a major capital flood defence scheme on the River Torne between 1985 and 1990, selected reaches of the main trapezoidal channel were enhanced. By winning spoil from the channel margins and from borrow pits in the floodplain, a more varied marginal zone was created which maximised the potential habitat for wetland plant communities and their associated fauna. Enhancement comprised bank re-profiling to create narrow wetland shelves (berms), shallow bays, channel margins of varying shape and depth and linear still ponds from the borrow pits. The 1990 planting programme comprised 11 macrophyte species and a total of 7740 individual plants. This paper reports on an initial study to evaluate the marginal habitat enhancements on the River Torne 5 years after completion of the project. Lack of pre-scheme data necessitated a space-time substitution; enhanced river margins were compared with neighbouring reaches that had undergone conventional floodbank repair and remained as trapezoidal channel sections planted with a standard, low maintenance seed mix. Marginal vegetation was surveyed and supported by measurements of the physical habitat at 10 enhanced and 10 conventionally-engineered reaches. The macrophyte surveys and the results from the cluster analysis and polar ordination indicate that enhanced and conventionally-engineered reaches are floristically distinct and that the enhanced reaches have a more varied macrophyte community. The results from the Mann–Whitney U-tests show that enhanced reaches have significantly higher values of wetland species diversity and equitability, percentage of wetland species, bank width and soil moisture and significantly lower bank angles. However, the correlation and linear regression analyses did not show any strong associations between the physical habitat and plant parameters. Copyright © 2000 John Wiley & Sons, Ltd.

Annual changes in summer patterns of mesohabitat distribution and associated macroinvertebrate assemblages

The summer distribution of distinct mesohabitats (gravel, sand, silt, Ranunculus beds and emergent vegetation patches) at four 100-m sections along the length of a lowland river was studied over 3 years, together with their associated faunal assemblages. Habitat proportions in the sections showed little change between 1995 and 1997 at the uppermost site, in contrast to major changes downstream, which included an increase in percentage cover of Ranunculus from 10% in 1995 to 50% in 1996. There did not appear to be a significant relationship between summer or winter discharge and the proportions of mesohabitat, but there were indications that stream macrophyte areas were high when the preceding winter flows were low. Multivariate analysis of the faunal/site matrix showed that the principal source of variation was the mesohabitat composition. Superimposed over this was a site component where the uppermost site was separated from the intermediate sites, which in turn were separated from the most downstream site. Annual changes in faunal composition appeared to be the least significant overall but this varied with site, with little difference at the uppermost site and clear differences between 1995 and the other two years at the other sites. Ranunculus beds showed the least change between years. The highest number of taxa was found in emergent or marginal vegetation at all sites, with Ranunculus and sand supporting the lowest number. Mean abundance was greatest in Ranunculus and gravel samples, intermediate in emergent vegetation and silt, and lowest in sand. In order to investigate the implications of habitat changes, mean abundance of taxa per mesohabitat per year was compared with total abundance in the 100-m section for which habitat information was available. The total mean abundance of macroinvertebrates per mesohabitat was greatest in 1995 but this is not reflected in the total abundance per section. Mesohabitat abundances were greatest in Ranunculus, silt and emergent vegetation in 1995 but these mesohabitats were poorly represented in the section. In 1996, Ranunculus increased in area and this was reflected in higher total abundance. In 1997, the mean number of animals per mesohabitat was less than in previous years and this resulted in low overall abundance for the reach. Thus, the changes in habitat proportions controlled annual changes in total reach abundance and composition. Such changes have implications for the interpretation of biomonitoring and river habitat survey results whereby misleading conclusions may be drawn without considering annual and spatial fluctuations in habitat proportions. Copyright © 2000 John Wiley & Sons, Ltd.

Divergent life history traits in two closely related species of Simocephalus (Cladocera: Daphnidae) inhabiting lentic environments in Venezuela

We describe life history tactics under laboratory conditions of two species of cladocerans of the genus Simocephalus. The populations live in two habitats with different characteristics. S. acutirostratus was isolated from a small temporary pool without fish. S. latirostris was found in the marginal vegetation of a reservoir with fish. Their life history was monitored for differences in traits such as clutch size, neonate size, age distribution, reproductive effort and adult survival. Our results show that S. acutirostratus (the larger-sized species) grows until it reaches the optimal foraging size and then begins to reproduce, while S. latirostris (the smaller-sized species) starts breeding before reaching the optimal foraging size, allocating energy mainly to reproduction. We explore the possibility that divergences in life history may arise as a response to environmental stress such as that produced by fishes.

The Possible Role of Volatile Secretions as Intra- and Interspecific Alarm Signals in Gyrinus Species

Gyrinid beetles have paired pygidial glands producing a secretion consisting of both a high molecular weight fraction, rendering them toxic for predatory fish, and usually also volatile fractions (8 of 10 north European Gyrinus species), which act as alarm signals. The presence of multi-species aggregations on the water surface raises the question whether the pygidial secretion of one species can elicit an escape response in other species. There were no differences in escape reaction to signals between two strongly scented species (G. aeratus and G. substriatus) and one species without obvious scent (G. opacus ). Furthermore, signals were received across the species borders, although only the two species emitting volatile components were able to alarm other individuals. The two scented species (G. aeratus and G, substriatus) often aggregate in large groups on open water, whereas the unscented species (G. opacus and G. minutus) rarely do so, but preferably occur singly or in loose groups among the emergent marginal vegetation of various standing waters. This suggests that the presence of alarm signals is correlated with behaviour and habitat choice. Furthermore, the putatively weak competitor G. opacus spends significantly more time below the water surface than G. substriatus, perhaps as a result of it lacking an alarm function. The occasional presence of individuals of the two unscented species in large aggregates of G. aeratus on open surface, may indicate that they may take advantage of the alarm signals produced by individuals of another species and as result escape approaching danger. It is not known whether G. opacus and G. minutus have lost the ability to produce volatile alarm substances or whether they never have had it. Comparisons with the ca 11000-year-old subfossil localities of G. opacus, however, indicate that at least from that date this species has inhabited localities similar to the present with respect to predation risks and other properties, i.e. mainly small, predatory fish-free waters.

The past, present and future of Llangorse Lake—a shallow nutrient-rich lake in the Brecon Beacons National Park, Wales, UK

1. This paper provides an introduction to the ecology and conservation status of Llangorse Lake, a shallow nutrient-rich lake in the Brecon Beacons National Park, Wales, UK. The lake has been designated as a Site of Special Scientific Interest in Britain and was recently proposed as an internationally important conservation site under European Community legislation.2. The water chemistry and biota are indicative of alkaline, nutrient-rich conditions. Water transparency is low and there is evidence of deoxygenation above the surface sediments.3. The epilithic diatom flora exhibits the most diverse assemblage of taxa. Cocconeis placentula and Achnanthes minutissima dominate the epiphytic diatom flora, whilst the surface sediment diatom assemblage is largely composed of planktonic taxa.4. Phragmites australis and Typha latifolia are the dominant components of the emergent stands of marginal vegetation, which is flanked on the open water side by floating-leaved plant communities. Several species of Potmogeton are recorded as parr of the diverse submerged flora.5. The open water and littoral zooplankton assemblages are composed of a small number of cladoceran and copepod species. In contrast, a diverse assemblage of macroinvertebrates with large numbers of individuals is recorded. Oligochaetes are the most numerous group but the Mollusca, Hirudinea, Malacostraca and insects are also well represented.6. Artificial enrichment, power boating and fishery management are highlighted as key considerations for the future conservation of the lake. A number of non-indigenous plant and animal species are reported.7. The organizations that can play a role in the future management of the lake are identified and a number of management planning initiatives are discussed. Copyright (C) 1999 John Wiley & Sons, Ltd.

Observations on aquatic Macrophyte Dynamics In the Reservoir Of The Balbina Hydroelectric Powerplant, Amazonas State, Brazil

Between 1991 and 1995 aquatic macrophyte composition was observed in the lower part of the reservoir of the hydroelectric power plant of Balbina (Amazonas, Brazil). After closure of the dam in 1987, vegetation cover - mostly Eichhornia crassipes - was high, but was not quantified. After 1990 it declined rapidly with a characteristic succession pattern: Eichhornia ® Vincularia + Cyperaceae ® Salvinia. The Cyperaceae, and many other less dominant species, were mostly associated with drift wood, produced by the decomposing, emergent forest. Comparison of the chemical data of the Uatumã river before the construction of the dam (1983) with those of later years (1989 - 1995) suggests that the succession was the result of a relatively mild and short period of eutrophication, followed by declining nutrient levels. Annual variation of water levels, followed by aquatic and terrestrial decomposition of the marginal vegetation, may allow for the maintenance of relatively productive vegetation belts along the shore lines of islands and inundated stream valleys.

Origin and trophic importance of detritus-evidence from stable isotopes in the benthos of a small, temperate estuary.

Ratios of 13C/12C and 15N/14N were measured in dissolved inorganic carbon (DIC), marginal vegetation, benthic macrodetritus (diameter > 1 mm) and selected invertebrate consumers in the Gamtoos estuary, South Africa to: (1) trace the provenance of benthic detrital deposits, and (2) determine the extent to which three abundant species of macroinvertebrates utilise this resource. DIC was strongly depleted in 13C with average δ13C values (-9.5±0.5‰) being typical of limnetic waters. Benthic detrital particles (δ13C-24.1±0.3‰) originated mainly from marginal vegetation (δ13C-25.7±0.3‰), but their slightly elevated carbon ratio suggests additional input from 13C-rich sources-possibly C4 plants cultivated on the floodplain. Populations of the fossorial ghost shrimp Callianassa kraussi, the bentho-pelagic amphipod Grandidierella lignorum and the epifaunal crab Hymenosoma robiculare together account for 96% of total benthic biomass in the upper regions of this estuary. Marked differences in trophic niches were evident among these three consumer species. Ghost shrimp (δ13C -32.5±0.3‰) foraged by filter-feeding on fine suspended particulate organic matter (δ13C-31.2±0.5‰). Amphipods (δ13C-28.0±0.6‰) utilised some benthic detritus but fed mainly on suspended material. Only the relatively rare crabs (δ13C-23.8±1.5‰) appeared to utilise benthic detrital particles to any significant extent. In the benthic consumer community of the upper Gamtoos estuary, suspension feeders make up 98% of biomass and thus clearly dominate over deposit feeders. This can be traced to the low contribution of higher plants (c. 13%) to overall carbon production, and detritus originating from macrophytes is consequently relatively unimportant in supporting invertebrate secondary production in this particular system.

Temporal constancy of faunal assemblages in 'mesohabitats' - Application to management?

Variations in the composition of faunal assemblages in streams are often associated with changes in areas of substrate. An observer walking along the bank finds these areas easy to distinguish and we have used these as our basic ecological unit. Essentially the unit is visually distinct from the bank and we have called it a mesohabitat to introduce a scalar dimension which the term biotope does not have and to distinguish it from microhabitats such as a leaf tip or stone surface and macrohabitats which might include the whole of a reach of river. We wished to test the hypothesis that mesohabitats support specific faunal communities which remain distinct and faithful throughout the seasons and to explore the use of such information in environmental assessment. 10 replicate faunal samples, together with associated depth and velocity measurements, were collected from each of 8 mesohabitats ('Ranunculus fast', 'Ranunculus slow', 'Silt', 'Nasturtium', 'Phragmites', 'Sand', 'Gravel fast' and 'Gravel slow') in spring, summer and autumn from a lowland weeded stream, the Mill Stream, in southern England. Analysis of variance indicated that both abundance and faunal richness varied significantly between and within mesohabitats. The most valiability was noted in 'sand', 'Ranunculus slow', and 'silt'. Variation between mesohabitats was significant for all seasons but there was a tendency for the differences to be greatest in spring samples. Multivariate techniques, DECORANA and CCA (canonical correspondence analysis) showed that in general the mesohabitats selected for study remained distinct from one another but that the degree of separation varied markedly with season. In spring when discharge was high mesohabitats showed considerable overlap on the basis of their faunal communities. As the year progressed and discharge declined so mesohabitats became more distinct. There is a continual expansion and contraction of available mesohabitat. Thus instream macrophyte beds increase at the expense of open water habitat, and marginal vegetation encroaches into the main flow. The stream therefore consists of a dynamic system of mesohabitats whose borders are best defined in low flow periods. The 'distinctness' of the mesohabitats meant that it was possible to determine their proportion along a 300 m reach of the Mill Stream. The application of these data is discussed. This information together with data on the 'quality' (species richness, faunal abundance, rarity) can be used to categorise a stream at reach level and assess the effect of management (flow changes, dredging, bank clearance and weed-cutting) on the distribution of mesohabitats and hence on the faunal assemblages associated with them. This could be considered a more useful scale than point sampling when considering fish and macrophytes as well as invertebrates.

Aquatic and marginal vegetation of the River Divor and its relation to land use

(1994). Aquatic and marginal vegetation of the River Divor and its relation to land use. SIL Proceedings, 1922-2010: Vol. 25, No. 4, pp. 2309-2315.

Initial observations on the effects of varying levels of deciduous bankside vegetation on salmonid stocks in Irish waters

Abstract. The effects of varying levels of deciduous bankside vegetation on salmonid stocks in Irish rivers were investigated. In summertime, when marginal vegetation limited the extent of incident light reaching the river bed, a marked decline in both juvenile salmon, Salmo salar L., and juvenile and adult trout, Salmo trutta L., numbers were observed relative to stocks in adjacent areas with a less dense canopy. This appears to be a countrywide phenomenon in all available salmonid habitats. In the case of both juvenile salmon and all trout numbers a correlation is evident between the extent of shade, as measured in terms of the reduction in aquatic vegetation, and fish numbers, which fall as shade levels increase. Data also suggest that the length of tunnelled channel, upstream of tunnelled sites electrofished, seems to influence the standing crop of juvenile salmon with numbers of these fish falling with increasing tunnel length. This relationship is not evident in relation to trout numbers.

A COMPARISON OF CYPRINID FISH POPULATIONS IN THREE LARGE TRANSKEI RESERVOIRS WITH RESPECT TO ABUNDANCE AND REPRODUCTION

Summary Gill net catches of fish in three turbid reservoirs (Lakes Xonxa and Lubisi on the Kei River system and Lake Umtata on the Mtata River) between 1982 and 1985 were dominated by two cyprinids the common carp, Cyprinus carpio, and smallmouth yellowfish, Barbus aeneus. Mean catch per unit effort of carp was low in all three reservoirs (5–6.5 fish 120 m net−1) while smallmouth yellowfish, absent from Lake Umtata, were abundant in Lakes Lubisi and Xonxa (62.9 and 131.5 fish 120 m net−1, respectively). Monthly changes in the gonadosomatic index and gonad state of carp as well as direct observations of spawning fish showed that egg laying in this species occurred only once rising water levels inundated marginal vegetation. Regional weather patterns characterised by heavy mid- and late summer rains lead to late, relatively unsuccessful spawning by carp and may account for their low abundance in local reservoirs. Growth of yellowfish in Lake Lubisi was poorer and size at sexual maturity smaller than in Lake...

Are Macroinvertebrate Assemblages in the Buffalo River, Southern Africa, Associated with Particular Biotopes?

Stream biotopes (habitat types) have commonly been defined using subjective recognition of hydraulic and substrate conditions. In this study the relationship between groups of macroinvertebrates and subjectively defined biotopes was examined at three sites in the upper, middle, and lower reaches of the Buffalo River. Biotopes which were recognised included riffles, leaf packs in riffles, and a waterfall (erosional biotopes); and stony backwaters, marginal vegetation and sediments (depositional biotopes). Over 100 taxa were identified from the three sites, and an hierarchical classification of 138 samples was prepared using TWINSPAN. The first division distinguished between the invertebrate assemblages of the upper reaches and those of the middle/lower reaches. In the upper reaches, the waterfall was differentiated from all the other stream biotopes. In addition, seasonal changes in assemblage composition were recognised. Divisions in the middle/lower reaches differentiated between biotopes. Of the 27 most common taxa, 13 occurred in a single biotope in more than 50% of the samples in which they had been collected. Clearest species-biotope associations were riffle-dwelling mayflies and caddisflies, and stone-dwelling planaria and ancylid gastropods. Invertebrate assemblages were not always associated with subjectively identified biotopes, particularly in the narrow headwaters, where different biotopes tended to be smaller and less distinct. However, associations of assemblages with biotopes were found in the middle/lower reaches. This study shows that stratified random sampling regimes can be useful tools in stream research if the scale at which biotopes are recognised is clearly defined.

MACROPHYTE STANDING CROP AND PRIMARY PRODUCTWITY IN SOME FLORIDA SPRING‐RUNS1

ABSTRACT: A survey of 31 Florida spring‐runs was conducted to estimate their submerged macrophyte standing crop and primary productivity. The standing crops of submerged vegetation were not significantly (p &gt; 0.05) correlated to either total nitrogen (r = 0) or total phosphorus (r =−0.20) concentrations, but standing crops were significantly (p &lt; 0.01) correlated to the percentage of the spring‐run's water surface shaded by marginal vegetation (r =−0.76). Maximum daily rates of primary productivity were positively correlated with average submerged macrophyte standing crops (r = 0.81; p &lt; 0.001) and inversely correlated with the degree of shading by marginal vegetation (r =− 0.43; p &lt; 0.05).

The distribution and ecology of the freshwater prawnCaridina singhalensis(Decapoda, Atyidae) endemic to Sri Lanka

ABSTRACTThe distribution and microhabitat preference of the freshwater prawn,Caridina singhalensis, endemic to Sri Lanka, was determined. The species is now restricted to a 10–15 km stretch of the Behihul Oya headwaters, its distribution being determined by physiography and agricultural development.Beds ofIsolepis fluitanssupport some 76% of the adult population, at mean monthly densities of around 210 individuals m−2.Aponogeton jacobseniibeds and roots of marginal vegetation beneath overhanging banks support about 12% each, at mean monthly densities of 41 individuals m−2. The prawn produces young all year round but a peak in the number of ovigerous females occurs from September to January. Only juveniles produced after the rains appear to survive in great numbers. The submerged roots of marginal vegetation form a nursery area for about 45% of the annual output of young. Conservation plans for this endangered species need to take account of the particular importance ofI. fluitansand root habitats toC. singhalensis.