Shallow Littoral Zone Research Articles

Groundwater discharge gives periphyton a competitive advantage over macrophytes

Lacustrine groundwater discharge (LGD) can significantly contribute to nutrient budgets of lakes. LGD usually occurs in shallow littoral zones, and thus, potentially affects periphyton and macrophytes. These benthic primary producers asymmetrically compete for resources, with periphyton and rooted macrophytes having an advantage in either light or sediment nutrient acquisition, respectively. LGD can advectively transport sediment phosphorus (P) into the water, supporting periphyton growth. We hypothesised that above a threshold level of sediment P concentrations, negative effects of periphyton shading exceed the positive effects of sediment P on macrophytes. We performed laboratory experiments comparing the growth of periphyton and a macrophyte (Stuckenia pectinata) at four different sediment P concentrations with and without simulated LGD. In LGD treatments, P mobilized from sediment promoted periphyton growth, and macrophyte growth rates negatively correlated with periphyton shading when experiments were performed with young plants. Macrophytes died when shading levels exceeded 75% of incident light. Our results confirm that LGD can significantly affect the competition between benthic primary producers at the expense of macrophytes. This process is supposed to be most relevant in oligotrophic water bodies or during periods of P deficiency in the open water, when advective transport of sediment P by LGD can prevent nutrient limitation in periphyton.

Amphibians in drained forest landscapes: Conservation opportunities for commercial forests and protected sites

Forest drainage has led to the loss of a considerable number of wetlands, especially in the northern hemisphere. In order to find ways to mitigate the loss of biodiversity in drained forests, we investigated two distinct conservation measures: (i) leaving drained peatlands for natural succession, i.e. protecting without active restoration, and (ii) constructing a variety of mitigation pools during ditch maintenance work in commercial forests. We tested the effectiveness of these conservation approaches for wetland biota, selecting brown frogs (Rana arvalis, R. temporaria) as our focal species. We found that ditches do not substitute natural floods as breeding habitats for brown frogs in protected peatlands. One of the main reasons was the reduction of sun exposure due to drainage-induced forest growth. However, secondary wetlands formed on ditches impounded by Eurasian beaver (Castor fiber) offered high quality reproduction sites for brown frogs. In commercial forests, the number of natural pools decreased due to ditch maintenance work, but the colonisation rate of brown frogs increased in cleaned ditches. The reproduction site selection of the two frog species differed – R. arvalis bred mainly in natural and constructed pools, while R. temporaria bred more frequently in ditches. Among mitigation pools, those with a shallow littoral zone were primarily used for breeding. Thus, the conservation measures are effective only if certain key assumptions are met. The success could stand on the considering of the habitat requirements of target species before creating mitigation habitats or presence of a restoration agent (like beaver) in protected sites modified by human activities.

Effects of water bugs on crustacean zooplankton in a shallow littoral zone

Interactions within the food chain are essential to ecosystem ecology. At the same time, very little is known of the trophic relationships between Heteroptera and zooplankton. The aim of this study was to examine the community structure of water bugs in the littoral zone of shallow lakes and their impact on crustacean zooplankton communities. A study on the effect of Heteroptera on zooplankton communities was carried out in the conditions of a laboratory experiment with two treatments: (1) aquariums in which the Heteroptera speciesIlyocoris cimicoidesandPlea minutissimawere present (control treatment) and (2) aquariums from which Heteroptera were removed (removal treatment). The decisive factor in reducing the number of crustacean zooplankton was most likely the presence of predatory Heteroptera in a composition typical of the shallow littoral of eutrophic lakes. Their impact is reflected in both a decrease in the species richness and abundance of crustaceans and a transformation of the size structure of the zooplankton. The results of the study are the first to suggest a substantial seasonal impact of water bugs on crustacean zooplankton in a shallow lake. The results of the experiments confirm that predatory Heteroptera exert considerable pressure on crustacean zooplankton during the spring and summer peaks in abundance.

Stimulation of epiphyton growth by lacustrine groundwater discharge to an oligo-mesotrophic hard-water lake

AbstractPeriphyton is a major contributor to aquatic primary production and often competes with phytoplankton and submerged macrophytes for resources. In nutrient-limited environments, mobilization of sediment nutrients by groundwater can significantly affect periphyton (including epiphyton) development in shallow littoral zones and may affect other lake primary producers. We hypothesized that epiphyton growth in the littoral zone of temperate oligomesotrophic hard-water lakes could be stimulated by nutrient (especially P) supply via lacustrine groundwater discharge (LGD). We compared the dry mass, chlorophyll a (chl a), and nutrient content of epiphyton grown on artificial substrates at different sites in a groundwater-fed lake and in experimental chambers with and without LGD. During the spring–summer periods, epiphyton accumulated more biomass, especially algae, in littoral LGD sites and in experimental chambers with LGD compared to controls without LGD. Epiphyton chl a accumulation reached up to 46 mg...

Storm-Induced Atlantic Herring (Clupea harengus) Egg Mortality in Baltic Sea Inshore Spawning Areas

During their spring migration, Atlantic herring (Clupea harengus) populations in the Baltic Sea rely on shallow transitional waters, such as estuaries, bays, and lagoons for spawning. Such inshore spawning grounds are ecologically important by providing suitable substrates for demersal egg deposition. These habitats are often highly impacted by multiple anthropogenic threats. Decades of eutrophication have caused a decline in depth distribution of submerged aquatic vegetation, the main herring spawning substrate in the Baltic Sea. Nowadays, spawning beds are limited to the shallow littoral zone (≤3 m depth). Accordingly, macrophytes are increasingly exposed to mechanic forcing due to storm-induced wave action. Generally, reproductive success and year class strength of the Western Baltic herring population is strongly determined by the survival of early life stages such as eggs and larvae in local nursery areas. However, explicit mechanisms by which local stressors might affect overall recruitment are currently not well understood. Hypothesizing that aquatic vegetation limited by water depth causes high herring egg mortality due to increased exposure to storm-induced hydrodynamics, we performed a combination of field studies investigating the impact of storm events on herring egg loss. Results of an egg loss experiment revealed a total egg loss of 29% in one single spawning bed during a storm event within the spawning season and the quantification of eggs attached to macrophyte litter on the shoreline emphasize the potential for regional weather extremes such as storm events to act as influential stressors for herring reproduction.

Input of organic matter to the Buor-Khaya Gulf (Laptev Sea)

We present results of study of organic matter in the coastal and bottom sediments of the Laptev Sea (Buor-Khaya Gulf). The study has shown the regularities of organic-matter distribution in the shelf zone adjacent to the glacial coast. The coast composed of a glacial rock complex supplies the largest amount of organic material to the sea as compared with other types of the coast. The average content of organic matter in these strata is ~2-3%. The bottom sediments of the shallow littoral zone are significantly depleted in organic carbon (0.1-0.3%) as a result of their active rewashing and the transition of finely dispersed material (mainly organic one) toward the deep sea zones. The content of organic carbon in the bottom sediments increases to 1-2% as the sea deepens to 7-10 m at 5-12 km from the coast. There are frequent local anomalies of organic-carbon contents (up to 4-5%) in the deltaic zones of the sea. The highest contents of organic carbon (up to 3%) have been found in the recent marine sediments in the central, relatively deep zones of the bays.

Effects of macrophyte development on the oxygen metabolism of an urban river rehabilitation structure

To compensate for fairway enlargements and the hydraulic impacts of navigation activity, an artificial rehabilitation structure was constructed in the urban, navigable River Spree in 2004. This wave-protected, shallow littoral zone proved to be highly effective in reducing vessel-induced waves and provided suitable conditions for the development of aquatic plants. However, in time it became less suitable for other aquatic organisms due to hypoxic conditions in late summer. This study aimed to comparatively calculate and analyze the oxygen balance of the rehabilitation structure and the main channel five years after the construction in 2009.In the rehabilitation structure, the production to respiration ratio ranged between 0.10 and 0.34 at the peak of vegetation density, while in the main channel in front of the rehabilitation structure it ranged between 0.67 and 0.86. Dense vegetation limited the water exchange and caused oxygen depletion. Thus, atmospheric oxygen input through the water surface and due to long-term water level changes became the most important supply processes for oxygen in the rehabilitation structure.Enhancing the oxygen supply to improve the suitability of the rehabilitation structure for other aquatic taxa requires an increase in water exchange with the main channel, e.g. by adaptively increasing the lateral connectivity.

Temperature‐ and light‐dependent ratio of energy gain to metabolic costs explains spatial and temporal habitat use of zooplanktivorous fish

AbstractUnderstanding the forces that drive habitat selection of species in communities is important in both ecology and evolution. In nature, species face variation in competition, predation and physical characters among habitats. Vendace (Coregonus albula (L.)) is a specialised zooplanktivorous fish predominantly using deeper water in lakes during summer, while roach (Rutilus rutilus (L.)) uses mainly the shallow littoral zone as well as the upper layer of the pelagic zone. To understand mechanisms behind habitat use of these species, I first conducted a predation experiment to investigate their sensitivity to predation by perch (Perca fluviatilis L.). Second, I performed a foraging experiment using different temperature and light treatments. I then used metabolic calculations to estimate energetic costs when foraging. I found no difference between species regarding sensitivity to predation. Vendace was the most efficient forager on zooplankton but also swam faster spending more energy compared to roach. Roach had a comparatively high metabolic rate in the lowest temperature, where their foraging efficiency was lowest. The energy gain ratio at 6°C was highest for vendace, while it was lowest for roach. In the highest temperature (18°C) and the lowest light level (1 lux), both species were similar in their energy gain ratio. The relative energy gain ratio provides a mechanism to explain habitat distribution for the two species. An increased understanding of the role of metabolism in combination with biotic interactions and habitat use may help to foresee effects of environmental change for different species.

Habitat choice and female preference in a polymorphic stickleback population

Background: A small pond, c. 90 years old, near Bern, Switzerland contains a population of threespine stickleback (Gasterosteus aculeatus) with two distinct male phenotypes. Males of one type are large, and red, and nest in the shallow littoral zone. The males of the other are small and orange, and nest offshore at slightly greater depth. The females in this population are phenotypically highly variable but cannot easily be assigned to either male type. Question: Is the existence of two sympatric male morphs maintained by substrate-associated male nest site choice and facilitated by female mate preferences? Organisms: Male stickleback caught individually at their breeding sites. Females caught with minnow traps. Methods: In experimental tanks, we simulated the slope and substrate of the two nesting habitats. We then placed individual males in a tank and observed in which habitat the male would build his nest. In a simultaneous two-stimulus choice design, we gave females the choice between a large, red male and a small, orange one. We measured female morphology and used linear mixed effect models to determine whether female preference correlated with female morphology. Results: Both red and orange males preferred nesting in the habitat that simulated the slightly deeper offshore condition. This is the habitat occupied by the small, orange males in the pond itself. The proportion of females that chose a small orange male was similar to that which chose a large red male. Several aspects of female phenotype correlated with the male type that a female preferred.

Depth-Related Effects on a Meiofaunal Community Dwelling in the Periphyton of a Mesotrophic Lake.

Periphyton is a complex assemblage of micro- and meiofauna embedded in the organic matrix that coats most submerged substrate in the littoral of lakes. The aim of this study was to better understand the consequences of depth-level fluctuation on a periphytic community. The effects of light and wave disturbance on the development of littoral periphyton were evaluated in Lake Erken (Sweden) using an experimental design that combined in situ shading with periphyton depth transfers. Free-living nematodes were a major contributor to the meiofaunal community. Their species composition was therefore used as a proxy to distinguish the contributions of light- and wave-related effects. The periphyton layer was much thicker at a depth of 30 cm than at 200 cm, as indicated by differences in the amounts of organic and phototrophic biomass and meiofaunal and nematode densities. A reduction of the depth-level of periphyton via a transfer from a deep to a shallow location induced rapid positive responses by its algal, meiofaunal, and nematode communities. The slower and weaker negative responses to the reverse transfer were attributed to the potentially higher resilience of periphytic communities to increases in the water level. In the shallow littoral of the lake, shading magnified the effects of phototrophic biomass erosion by waves, as the increased exposure to wave shear stress was not compensated for by an increase in photosynthesis. This finding suggests that benthic primary production will be strongly impeded in the shallow littoral zones of lakes artificially shaded by construction or embankments. However, regardless of the light constraints, an increased exposure to wave action had a generally positive short-term effect on meiofaunal density, by favoring the predominance of species able to anchor themselves to the substrate, especially the Chromadorid nematode Punctodora ratzeburgensis.

Spatial pattern of hydrolittoral rock encrusting assemblages along the salinity gradient of the Baltic Sea

Abstract This study compared the diversity parameters and structures of encrusting assemblages in two habitats situated at two levels of shallow rocky shore: hydrolittoral and littoral along the Baltic Sea system. We investigated the variability and level of distinctiveness of the hydrolittoral encrusting fauna based on species biodiversity and distribution, and compared these features with those of communities inhabiting the adjacent shallow littoral zone (3-m depth). Structural similarities and differences between the encrusting assemblages from adjacent hydrolittoral and littoral zones were studied within 14 locations distributed along the northern coastline of the Baltic Sea. Multivariate analysis indicates that salinity had the greatest influence on the structure of the investigated assemblages. Most of the observed hydrolittoral assemblages contained the same species as the littoral zone. This result indicated a shared common species pool with similar large-scale patterns of species distributions with some variability in the dominating species between zones. The similarity between species composition of the hydrolittoral and littoral assemblages decreased with increase of salinity. Additionally, with higher species richness and the occurrence of marine specialists adapted to hydrolittoral conditions, the role of the rock size in the frequency of species occurrence and assemblage diversity was less significant.

Autochthonous production in shallow littoral zones of five floodplain rivers: effects of flow, turbidity and nutrients

Summary Comparative research and generalisations in lotic ecology are challenged by the dynamic hydrology of fluvial systems. The aim of this study was to understand more fully how factors such as light, nutrients and flow can predict variation in autochthonous production and algal biomass. We measured seasonal changes in percent bankfull discharge, inorganic nutrient concentrations, turbidity, instream primary production, respiration and algal biomass in the littoral zone of five floodplain rivers in one temperate and two tropical regions of the Western Hemisphere. The Brazos, Guadalupe and Neches rivers are in Texas, while the Tambopata River is in Peru and the Cinaruco River in Venezuela. Our study rivers represented a range of hydrological regimes, turbidity levels and nutrient concentrations. Flooding patterns were more seasonal in the tropical rivers than in the (temperate) Texas rivers. Inorganic nutrient concentrations were higher in the temperate rivers, probably due to anthropogenic nutrient loading. Turbidity was higher following periods of high flow in the Brazos, Tambopata and Guadalupe rivers than in the Neches and Cinaruco rivers. Littoral zones in the sediment‐laden Brazos and Tambopata rivers became heterotrophic during periods of high discharge, while littoral zones in the Guadalupe, Neches and Cinaruco rivers were consistently autotrophic. Regression tree analysis suggested that algal production and biomass in the water column responded more strongly to seasonal changes in nutrients and temperature than to turbidity, while benthic algae responded more strongly to turbidity. Our findings suggest that during periods of high flow and turbidity in rivers containing fine sediments, autochthonous production is limited and terrestrial‐based organic matter may assume greater importance in the aquatic food web.

Effect of hydrodynamic disturbances on the biomass and architecture of the freshwater macroalga Chara globularis Thuill

The effect of wave-induced disturbances in the shallow littoral zone on Chara globularis individuals was studied in three populations located in three different lakes in Poland. In addition, an experiment was performed to check if the observed responses were confirmed in vegetative offspring grown ex situ. Individuals from wave-exposed sites were lighter than those from sheltered ones; they had more ramets and lateral branches. The main axes, branches, internodes and branchlets were shorter. The shortening of the main axis was a statistically significant response in each population. The reduction of biomass of thalli and shortening of branchlets under wave-exposed conditions was statistically significant only in part of the populations. The differences in underground biomass, number of branches, length of internodes and presence of holdfasts among individuals from wave-exposed and sheltered sites in the populations showed inconsistent trends between populations. This suggests a location-specific nature of some of the morphological variations. The vegetative offspring (new ramets obtained ex situ) from the fragments of individuals growing at wave-exposed sites were lighter, smaller, had shorter axes in a statistically significant way in comparison with the offspring from sheltered sites. The observed growth trend was the same as in the case of parent individuals. Hence the response noted in the parent population was confirmed in vegetative offspring. For all the remaining features, differences were more pronounced between new ramets than between parent individuals. Hence, in situ, the phenotypic responses to other environmental factors overlapped with the disturbance-related responses.

Lake bathymetry from Indian Remote Sensing (P6‐LISS III) satellite imagery using artificial neural network model

AbstractThe remote sensing technique provides a rapid and relatively inexpensive means of identifying silted areas in large water bodies, in order that desilting activities can be effectively conducted. This study developed lake bathymetry for a selected lake system (Akkulam–Veli Lake, Kerala, India) from the Indian Remote Sensing (IRS P6‐LISS III) satellite imagery, using an artificial neural network (ANN) model. The water depth was measured for 17 months at different points in the lake on the same date of overpass of the IRS satellite. The satellite imageries obtained for 12 December 2007 and 16 February 2009 were identified as cloud‐free images. ANN models were developed with the four input series of radiance values from green, red, NIR and MIR bands observed for the satellite imagery obtained on 12 December 2007 at the sampling sites, with actual water depth measurements also being taken on the same date. A three‐layered feed forward neural network with back propagation training algorithm was developed for this study. To train the model, it was run several times by changing the number of neurons, learning rate and the momentum constants until the mean square error was minimum. When the number of neurons is increased to 35, and the logsig function is used as ANN transfer function, the error becomes minimum. To test the model, the developed ANN was run for a new set of input from the satellite imagery taken on 16 February 2009. Comparing the predicted and measured values for the same sites for the same day, it was found that the model is best suited for predicting water depth using ANN and the radiance values for four bands of IRS satellite imagery. The results of this study indicated that, for the shallow lake with lower depth, the difference between the actual and predicted value was considerable. In contrast, this was not the case where the lake water depth was greater, indicating an increased prediction accuracy with ANN with increasing depths for shallow lakes. A bathymetry map prepared with ANN indicated only the lake shoreline, as well as the shallow littoral zones. The approach used in this study requires further refinement, including further of the model based on using more field measurements to obtain a better bathymetry map.

Dreissenid mussels enhance nutrient efflux, periphyton quantity and production in the shallow littoral zone of a large lake

Dreissenid mussels are notorious invasive organisms whose establishment is associated with large, ecosystem-scale changes to patterns of productivity in aquatic systems. We investigated how localized impacts of dreissenids affect the potential of littoral substrates to support primary and secondary production using in situ incubations and comparisons of natural mussel-colonized and mussel-free substrates in the littoral zone of a large, shallow lake. We compared dreissenid-colonized and dreissenid-free substrates in terms of nutrient balance, surface area, periphyton loads as well as benthic primary production and respiration rates. Dreissenid-colonized substrates acted as sources of dissolved nutrients to the water column, with mussel mass-specific rates of dissolved phosphorus and ammonia excretion averaging 7.2 ± 5.6 (mean ± SD), and 92.6 ± 64.7 μg/g mussel shell free dry mass/h, respectively. Mussel-colonized substrates also had higher surface area, and supported approximately double the amount of periphyton and organic matter loads compared to mussel-free substrates, as well as having higher rates of primary production and community respiration. We suggest that the localized effects of dreissenids can play an important role in changing whole-ecosystem production patterns, with the extent of dreissenid impacts strongly dependent on lake size and morphometry.

Diet composition of smelt Osmerus eperlanus (Linnaeus) in brackish near‐shore ecosystem (Eru Bay, Baltic Sea)

AbstractWe present new data on seasonal diet composition of subadult and adult smelt Osmerus eperlanus in near‐shore areas of the eastern Baltic Sea (Gulf of Finland). These findings elucidate the food web structure and the ecological role of smelt in the shallow littoral zone. The seasonal diet composition and feeding activity of adult and subadult smelt in the coastal area of the Gulf of Finland were studied by examining the stomach contents of 1397 individuals collected from April to December 2009. Amphipods and mysids were the most common prey. Piscivorous feeding was also very important, particularly in larger smelt during August–October. Seven fish species were recorded in the stomach contents; gobies and sticklebacks dominated. Smelt preyed also on isopods, cladocerans, decapods, molluscs, insects, insect larvae as well as polychaete annelids. Prey composition varied monthly and depended on smelt size. The higher proportion of empty stomachs was observed during June, September and October.

Stream inflow and predation risk affect littoral habitat selection by benthic fish

Summary1. We examined small, fishless headwater streams to determine whether transport of macroinvertebrates into the littoral zone of an oligotrophic lake augmented food availability for Cottus asper, an abundant predatory fish in our study system. We sampled fish and macroinvertebrates during the recruitment and growth season of 2 years, either monthly (2004) or bi‐monthly (2005), to observe whether stream inputs increased prey availability and whether variation in total macroinvertebrate biomass was tracked by fish.2. Observations from eight headwater streams indicated that streams did not increase the total macroinvertebrate biomass in the shallow littoral zone at stream inflows, relative to adjacent plots without stream inputs (controls). The taxonomic composition of stream macroinvertebrates drifting toward the lake differed from that in the littoral lake benthos itself, although there was no evidence of any species change in the composition of the littoral benthos brought about by stream inputs.3. Although streams made no measurable contribution to the biomass or taxonomic composition of the littoral macroinvertebrate benthos, there was substantial temporal variation in biomass among the eight sites for each of the (n = 7) sample periods during which observations were made. Variation in total biomass was primarily a function of bottom slope and benthic substrata in the lake habitats. Dominant taxonomic groups were Baetidae, Ephemerellidae (two genera), Leptophlebiidae, Chironomidae (three subfamilies) and Perlodidae, although we did not determine the specific substratum affinities of each taxon.4. Mixed effects linear models identified a significant interaction between macroinvertebrate biomass and plot type (stream inflow vs. control) associated with fish abundance. Across the observed range of macroinvertebrate biomass, fish showed a significant preference for stream inflows, but more closely tracked food availability in the controls. For young‐of‐the‐year (YOY), a negative effect of temperature was also included in the model, and we observed lower temperatures at stream inflows. However, abundance of predatory adults affected habitat selection for YOY. Lake‐bottom slope also accounted for variation in abundance in both fish models.5. Our results suggest that the effect of fishless headwater streams on downstream fish may not always be through direct delivery of food. In this study system, fish preferred stream inflow plots, but this preference interacted with macroinvertebrate biomass in a manner that was difficult to explain. For YOY, predation risk was related to the preference for stream inflows, although the specific factor that mitigates predation risk remains poorly understood.

Changes in community composition, carbon and nitrogen stable isotope signatures and feeding strategy in epilithic aquatic nematodes along a depth gradient

Periphyton is an important component within the littoral zones of lakes, but it is known to vary dramatically on small (cm–m) and large (km) spatial scales, showing differences in composition and abundance. Until relatively recently, changes in periphyton composition along depth gradients have not been studied sufficiently and the response of small meiobenthic invertebrates inhabiting the periphyton to vertically changing environmental conditions such as light are poorly understood. To investigate the changing community composition of epilithic (on stones) nematodes along a depth gradient, we conducted a field study at Lake Erken, Sweden, with the specific objective to investigate whether changes in periphyton composition (algae, bacteria) are reflected in changing nematode feeding types and what the consequences are for nematodes and their resource consumption. We analysed the abundance, species composition and feeding type distribution of epilithic nematodes along 11 depth levels, from 5 to 300 cm water depth. Our study resulted in the first measurements of carbon (13C) and nitrogen (15N) stable isotopes in free living nematodes in lakes. Nematode community composition and feeding type distribution exhibited dramatic changes along the depth gradient. Nematode feeding types changed from a dominance of algae-feeding species in the shallow littoral zone to one of bacteria-feeding species in the deep littoral zone. The 13C stable isotope signatures of nematodes and the small (<20 μm) periphyton fraction changed with increasing depth, with nematodes shown to feed on this small periphytic component. Nematodes were identified as primary consumers by means of trophic level calculations based on 15N stable isotopes.

The propagule supply, litter layers and canopy shade in the littoral community influence the establishment and growth of Myriophyllum aquaticum

The exotic macrophyte species Myriophyllum aquaticum (Vell.) Verdc. is widely dispersed, mainly through stem fragments and rhizomes, in shallow littoral zones in which uniform canopy conditions are created by the pre-existing resident wetland plant species. Therefore, interactions between these two types of asexual propagules and the resident species may occur during the early establishment and growth phases of M. aquaticum. We tested the hypothesis that the establishment and growth of M. aquaticum are affected by three factors: the number of propagules supplied, the presence of a standing biomass of Carex japonica Thunb. and the presence of thick litter layers in the littoral zone. M. aquaticum rhizomes and stem fragments were introduced into a standing biomass of C. japonica in mesocosms using two types of sediments (littoral and sand-clay) and two propagule supply levels. After 4 months, the survival rates of both the rhizomes and stem fragments were high under all of the experimental treatments. The propagule supply positively affected the survival rates and growth of M. aquaticum. The survival of the rhizomes was unaffected by the presence of either C. japonica or littoral sediments, whereas the survival of the stem fragments was reduced by the presence of C. japonica. The presence of litter layers is a primary factor facilitating the growth of M. aquaticum propagules because of the high nutrient content of the litter. In addition, the presence of the standing C. japonica biomass and newly growing Eleocharis yokoscensis (Franch.et Sav.) Tang et Wang (a small ruderal species that, unexpectedly, grew rapidly following the removal of the standing biomass of C. japonica) reduced the growth of the M. aquaticum stem fragments and rhizomes, respectively. Our findings suggest that the loss of vegetative cover resulting from intense cattle herbivory and other factors in littoral zones may accelerate the invasion of M. aquaticum. An effective approach for preventing M. aquaticum invasion is to reduce the propagule supply and prevent propagules from dispersing.

Substratum associations and depth distribution of benthic invertebrates in saline Walker Lake, Nevada, USA

Walker Lake, a terminal salt lake in western Nevada, is undergoing rapid changes because of falling lake level and rising salinity, affecting the potential habitat of benthic invertebrates that supply food to native fish and birds. Benthic invertebrate surveys were conducted within different substratum size classes and macrophyte beds in the nearshore littoral shallows ( 10 m) zones of the lake. Samples were dominated by the chironomid midges Cricotopus ornatus and Tanypus grodhausi; the damselfly Enallagma clausum; and an oligochaete worm of the genus Monopylephorus. Midges showed distinct depth preferences, with Cricotopus found primarily in the shallow littoral, and Tanypus found in the lower littoral and profundal regions. Enallagma occurred throughout the littoral region but was reduced in abundance below 10 m. Cricotopus and Enallagma were most abundant on cobble rock substratum and macrophytes. Sand and small gravel substrata supported few invertebrates except oligochaetes, which were most common in shallow littoral areas. The extent of Ruppia beds was determined using hydroacoustic sounding and showed that these beds were most well-developed in a zone from 1.25 to 5 m depth. The estimated area of productive shallow littoral zone habitat at different lake levels showed that coverage was lowest near the current surface elevation. Rising lake levels would result in expansion of suitable habitat area, and while falling levels could also expand nearshore habitat, this would likely occur on areas of poorer substratum quality and under high salinities that may inhibit growth.