Phytoplankton assemblage at equilibrium in large and deep subalpine lakes: a case study from Lago Maggiore (N. Italy)

In 1996, we studied the phytoplankton seasonal succession in Lago Maggiore (N. Italy) through weekly sampling. Such a frequency enabled us to evaluate the changes of the phytoplankton assemblage in the light of the equilibrium and non-equilibrium theories. The distinct phases of changing of the species composition were identified separating the samples by means of cluster analysis and non-metric multidimensional scaling (NMDS). We recognised well distinct phytoplankton associations, whose seasonal succession followed a clear cyclic path throughout the year, with spring and summer phases respectively characterised by a rapid turn-over of the assemblages and by a relative stability. Moreover, we observed an increase of species number and Shannon-Wiener diversity during the spring, followed by a summer decline of the diversity in spite of an unchanged species number. Because of the dominance of the same few species for about two months during summer, coupled with small fluctuations of the total biomass, we could identify the summer assemblage as a steady state assemblage. The aim of the present contribution, although describing the whole seasonal succession, is to draw the attention towards the species composition at the steady state, taking into account the functional properties of the species involved. The possible role of the metalimnetic niche in selecting a particular summer assemblage in deep and large lakes will be discussed.     

Solar heating and its influence on mixing in ice-covered lakes

SUMMARY 1. The influence of solar heating on the formation of temperature and density profiles of ice-covered lakes is considered.
2. Mathematical models are derived to quantify the effect of solar heating on vertical stability and its dependence on critical parameters,
3. Solar heating is shown to be able to account for the thermal structure and mixing patterns of certain ice-covered lakes.
4. The importance of convective mixing is discussed in relation to the distribution of phytoplankton and nutrients.     

Effect of vertical mixing on the vertical distribution of copepods in coastal waters

The vertical distribution of copepod adult stages and naupliihad been studied in a coastal water before and during homogenizationof the water column due to wind. When the water column was stratified,the vertical distribution of the zooplankton was also stratified,as is generally described in the literature. During homogenizationof the water column, different patterns of vertical distributionwere observed. The less active nauplius stage was evenly distributed,even though adult stages always exhibited a stratified distribution.Oithona similis and Microsetclla norvegica exhibited the samedistribution in stratified and unstratified water columns. Temoralongicomis and Pseudocalanus sp. were deeply distributed ina less turbulent area. The causes of these changes are discussed.     

on the vertical distribution of microorganisms in lake baikal during spring deep-water renewal

The vertical distribution of microorganisms during spring deep-water renewal in Lake Baikal was studied. The downward advection of trophogenic waters was found to create conditions for the extensive growth of microorganisms capable of decomposing and mineralizing organic carbon, nitrogen, and phosphorus in deep water layers. These processes occur annually at spring thermal bars near the underwater slope of Lake Baikal, whereas in its pelagic zone, the deep intrusions of waters rich in organic material are observed only in the years when enhanced deep-water renewal is accompanied by a high spring yield of phytoplankton.     

Lichen diversity on stumps in relation to wood decay in subalpine forests of Northern Italy

Although several studies demonstrated the importance of dead wood for lichen conservation in N-Europe and N-America, the lichen biota on dead wood was poorly studied in the Alps, where stumps represent one of the main available dead wood type. This work aims at evaluating species richness and composition of lichens in relation to the decay of stumps in subalpine forests of the Italian Alps. Differences in species richness between three decay stages were tested using a one-way ANOVA, while the pattern of species composition was evaluated with non-metric multidimensional scaling and an Indicator Species Analysis. Overall, 69 species were found and wood decay proved to be an important factor influencing lichen communities on stumps in our subalpine forests. Despite the fact that the mean number of species per stump did not differ between the three decay stages and lichen communities broadly overlapped, a main pattern of species turnover was identified across wood decay process as well as some indicator species for each decay stage. During the decay process, lichen communities change functional and ecological composition as an adaptive response to the continuous change of substrate. Since stumps host several nationally rare species, which are related to different stages of decay, they could have a relevant role in lichen conservation in managed forests where other types of dead wood are normally absent.     

UV damage in shallow lakes: the implications of water mixing

The plankton of shallow lakes is expected to experience increasingexposure to ultraviolet radiation (UVR) within the next decades.In addition to stratospheric ozone depletion, decreases in dissolvedorganic carbon (DOC), caused by climate warming or acidification,will result in greater UVR penetration in many lakes. Circulationpatterns induced by moderate winds force the plankton of shallowlakes to be cycled through the whole water column. This translatesinto higher doses and fluctuating dosage rates. Here, we reportthe results of a series of ‘in situ’ UV exposureexperiments in which two zooplankton crustaceans (one capableof photorecovery and the other not) were incubated either atfixed depths or under simulated mixing. Our main goal was toassess the extent to which the results from static incubationscould be extrapolated to vertically moving zooplankton. Staticincubations accurately predicted the survival of moving individualsfor the species without photorecovery, but failed to do so forthe species capable of photorecovery. The fluctuations in dosagerate for high doses tended to increase the survival of the specieswith photorecovery. We contend that neglecting the effects ofvertical mixing may result in a biased perception of UVR effectsin shallow lakes.     

Near-surface mixing and pronounced deep-water stratification in a compartmentalised, human-disturbed atoll lagoon system

Palmyra Atoll has four partially isolated lagoons up to 50 m in depth, each with complex and variable bottom topographies. Measurements of depth, temperature, salinity, turbidity and dissolved oxygen (DO) revealed a well-mixed shallow surface layer (0–10 m depth) and below that pronounced stratification of DO in the absence of a pycnocline. Turbidity increased in a step-like manner at ~25 m depth, at the oxycline. For all deep sections of the lagoon (>30 m), DO declined uniformly to 0% saturation. As determined from filtration, mass of particulates was independent of depth. Surface mixing and deep-water stratification are both stable at different temporal scales, including day versus night, daily, weekly and annually. We suggest that lagoon circulation is represented by a shallow, westward-moving surface layer of well-to-partially mixed water with high DO and low turbidity, underlain by a relatively static and temporally stable layer with low to zero DO and elevated turbidity. This is the first report of such conditions within a deep lagoon system, and only the second report of anoxic conditions in any such system. In deep-water, stable euxinic conditions reflect bottom topography, with dysoxic and anoxic water being constrained within silled basins. The occurrence and depth of large volumes of sediment-laden and dysoxic/anoxic water need to be considered in management proposals designed to increase water flow through the lagoon. These novel water column conditions most probably arose as a consequence of military construction work, consistent with published reports of profound changes to the atoll during 1940–1945. If so, they highlight the need to better understand the possible consequences of cutting channels and modification of lagoon flow at many atolls across the central Pacific Ocean.     

Fish assemblages in deep Italian subalpine lakes: history and present status with an emphasis on non-native species

We provide a comprehensive overview of the status of fish assemblages in eight Italian subalpine lakes using multiple data sources: historical records, standardised sampling and fishery catch statistics. Thirty-four fish species were recorded, twenty of which are native, seven introduced historically and another seven introduced recently. The number of fish species in individual lakes has increased by between 25 and 65% from 1855 to the present, with the increase being higher in the larger lakes. Non-native species contributed between 4.0 and 71.5% to standardised catches by numbers and between 4.8 and 65.1% by biomass. Roach Rutilus rutilus and pumpkinseed Lepomis gibbosus dominated electrofishing and benthic catches by number, while open-water catches were dominated by the coregonids European whitefish Coregonus lavaretus and gangfish Coregonus macrophthalmus. In terms of biomass, electrofishing catches were dominated by crucian carp Carassius carassius, largemouth bass Micropterus salmoides and wels catfish Silurus glanis, benthic catches were dominated by roach, European whitefish and crucian carp, and open-water catches by coregonids. Commercial fisheries are important ecosystem services provided by subalpine lakes, with their harvests now dominated by non-native species. To deliver effective biodiversity conservation and ecosystem services provision, the prioritisation of targets and interventions of management of non-native species is needed.     

Effect of submergence on translocation,starch content and amylolytic activity in deep-water rice

Submergence induces rapid internodal elongation in deep-water rice (Oryza sativa L. cv. Habiganj Aman II). We investigated the metabolic activities which help to support such fast growth. Three days of submergence in water under continuous light led to the mobilization of 65% of the starch from those regions of rice internodes which had been formed prior to submergence. Disappearance of starch was accompanied by a 70-fold enhancement of amylolytic activity. Similar increases in amylolytic activity were detected in response to ethylene and gibberellic acid. Submergence also caused a 26-fold increase in the translocation of newly synthesized photosynthetic assimilates from the leaves to the internodes and younger regions of the culms. These physiological processes are likely to provide the metabolic energy required for internodal elongation in response to submergence.Abbreviation GA₃ gibberellic acid     

Short-time effects of land-use changes on O-horizon in subalpine grasslands

Due to economic needs, a process of abandonment of unfavourably situated areas and intensified cultivation on favoured areas has taken place in the alpine region. This abandonment not only means the loss of a cultural landscape based on centuries of care but also interference with an established ecological balance. We wanted to investigate the impact of land-use, such as mowing, fertilisation and irrigation and the effects of abandonment on the O-horizon type, quantity and quality. Therefore, we selected areas with different types of land use but with equivalent site conditions in the Passeier Valley (Bolzano-South Tyrol province) for investigation. The areas studied were as follows: an organically fertilised and irrigated, intensively managed hay meadow mown once a year; an organically fertilised, intensively managed hay meadow mown once a year; a lightly managed hay meadow mown every second year; a lightly managed hay meadow mown every 3–4 years; an area densely covered with dwarf shrubs abandoned 10 years ago; and a reforested area abandoned 30 years ago. These selected areas represent different intensities of land use or stages of succession. On these areas, vegetation, phytomass, litter quantity, humus type, O-horizon quantity, litter decomposition and C and N storage, were analysed in detail. Abandonment causes the percentage of coverage with grasses and herbs to decrease from 100 to 10%, while lignified species increase from 0 to 90%. This leads to growth of the phytomass pools, which in turn causes an increase in the annual litter quantity (from 117 to 525 g m⁻²). Significant changes in litter quality in the case of reduced use or abandonment increases the litter’s resistance to decomposition, decreasing overall decomposition. Litter quantity and quality then have a significant impact on the O-horizon quantity. This increases strongly with decreasing intensity of land use or longer times since abandonment (115–1,180 g m⁻²). This leads to a significant increase of C and N storage and a change in O-horizon type ranging from no definite O-horizon or mull O-horizons on regularly used areas to moder on abandoned areas.     

The effects of vertical mixing on a phytoplankton community: a modelling approach to the intermediate disturbance hypothesis

1.  Connell's (1978) intermediate disturbance hypothesis (IDH) has been proposed as one explanation of why diversity is often highest at intermediate levels of disturbance. We used a model phytoplankton responses to environmental change (PROTECH) to investigate the validity of this hypothesis.
2. In a simulated phytoplankton assemblage of eight species, we found that the relationship between the increased intensity of a single forcing event and diversity was described by a positively skewed curve.
3. A progressive increase in forcing frequency introduced a sharp decrease in diversity at a threshold frequency. However, the highest diversity values were found at an intermediate frequency of disturbance.
4. We described the shape of this breakpoint response as like a 'cliff' and reconcile it with multiple stable-point theory. It is argued that the IDH should possibly be represented by this 'cliff' relationship, which may be applied to (or encourage the re-examination of) many previous studies.     

The effects of different oxygen regimes on the feeding and vertical distribution of Nephelopsis obscura (Hirudinoidea)

The feeding rates and vertical distribution of the freshwater leech Nephelopsis obscura, fed on different densities of third instar Chironomus riparius, were determined in the laboratory at three different constant dissolved oxygen regimes (10%, 100%, 300% saturation) at 15 °C. The rate of feeding increased linearly with prey density up to maximal field prey densities, with no satiation response. Analysis of covariance showed that the rate (slope) at which prey were consumed was not significantly different among oxygen regimes but the amount eaten at any given prey density was higher in higher dissolved oxygen saturations. Analysis of the vertical distribution of N. obscura at different oxygen saturations and prey densities showed a higher proportion of N. obscura on or in the mud substrate compared to the water column at 100% and 300%. No correlation was found between prey density and the vertical distribution of N. obscura, but the numbers of prey eaten were correlated with the proportion of N. obscura on the mud substrate. These results indicate that higher percentage dissolved oxygen regimes increase the feeding of N. obscura and are of potential importance in explaining the seasonal depth distribution and microdistribution patterns recorded for N. obscura.     

Climatic effects on the phenology of lake processes

Populations living in seasonal environments are exposed to systematic changes in physical conditions that restrict the growth and reproduction of many species to only a short time window of the annual cycle. Several studies have shown that climate changes over the latter part of the 20th century affected the phenology and population dynamics of single species. However, the key limitation to forecasting the effects of changing climate on ecosystems lies in understanding how it will affect interactions among species. We investigated the effects of climatic and biotic drivers on physical and biological lake processes, using a historical dataset of 40 years from Lake Washington, USA, and dynamic time‐series models to explain changes in the phenological patterns among physical and biological components of pelagic ecosystems. Long‐term climate warming and variability because of large‐scale climatic patterns like Pacific decadal oscillation (PDO) and El Niño–southern oscillation (ENSO) extended the duration of the stratification period by 25 days over the last 40 years. This change was due mainly to earlier spring stratification (16 days) and less to later stratification termination in fall (9 days). The phytoplankton spring bloom advanced roughly in parallel to stratification onset and in 2002 it occurred about 19 days earlier than it did in 1962, indicating the tight connection of spring phytoplankton growth to turbulent conditions. In contrast, the timing of the clear‐water phase showed high variability and was mainly driven by biotic factors. Among the zooplankton species, the timing of spring peaks in the rotifer Keratella advanced strongly, whereas Leptodiaptomus and Daphnia showed slight or no changes. These changes have generated a growing time lag between the spring phytoplankton peak and zooplankton peak, which can be especially critical for the cladoceran Daphnia. Water temperature, PDO, and food availability affected the timing of the spring peak in zooplankton. Overall, the impact of PDO on the phenological processes were stronger compared with ENSO. Our results highlight that climate affects physical and biological processes differently, which can interrupt energy flow among trophic levels, making ecosystem responses to climate change difficult to forecast.     

Plant response traits mediate the effects of subalpine grasslands on soil moisture

* In subalpine grasslands, changes in abiotic conditions with decreased management intensity alter the functional composition of plant communities, leading to modifications of ecosystem properties. Here, it is hypothesized that the nature of plant feedbacks on soil moisture is determined by the values of key traits at the community level. * As community functional parameters of grasslands change along a gradient of land uses, those traits that respond most to differences in abiotic conditions produced by land use changes were identified. A vegetation removal experiment was then conducted to determine how each plant community affected soil moisture. * Soil moisture was negatively correlated with community root length and positively correlated with canopy height, whereas average leaf area was associated with productivity. These traits were successfully used to predict the effects on soil moisture of each plant community in the removal experiment. This result was validated using data from an additional set of fields. * These findings demonstrate that the modification of soil moisture following land use change in subalpine grasslands can be mediated through those plant functional traits that respond to water availability.     

Nutrient and carbon relations in subalpine dwarf shrubs after neighbour removal or fertilization in northern Italy

Two subalpine dwarf-shrub heath communities with differing levels of soil nutrient availability were subjected to a 3-year experimental manipulation, including nutrient addition or removal of one of the two co-dominant species from each community. The main objective of our study was to assess the relative importance of interspecific competition versus nutrient limitation in relation to soil fertility. We also aimed to investigate if and to what extent current-year shoot size, leaf-based rates of net photosynthesis and foliar nutrient status accounted for the observed changes in the aboveground biomass of the shrubs. At the end of the experiment, neighbour removal increased the aboveground biomass of all shrubs, especially in the more fertile community, while fertilization did not. We concluded that: (1) competition is more effective than nutrient limitation in structuring the vegetation of subalpine heathlands; and (2) competition intensity is stronger in the more fertile community. The observed patterns of variations in aboveground biomass were not consistently related to net photosynthetic rates, size of individual shoots and foliar nutrient status. Hence, we also concluded that the growth response of dwarf shrubs to altered environmental conditions is primarily determined by developmental plasticity.     

Diel and vertical variability of seston food quality and quantity in a small subalpine oligomesotrophic lake

Vertical profiles of seston food quality and quantity were measuredin subalpine Castle Lake for particulate carbon, chlorophylla, fatty acids and phosphorus in addition to abiotic parametersincluding water temperature and dissolved oxygen levels. Fieldand laboratory incubation experiments were employed to manipulateDaphnia rosea growth environments. Sestonic eicosapentaenoicacid (EPA) content was much lower, whereas carbon to phosphorus(C:P) ratios were much higher in the epilimnion compared tothe deep-water chlorophyll maximum (16–20 m). In a temperature-controlledincubation, Daphnia grew faster when fed seston from the deep-waterchlorophyll maximum. In in situ cage incubations, D. rosea grewfastest in the epilimnion. Daphnia rosea in a migrating treatmentexhibited intermediate growth rates between the ones for epilimnionand hypolimnion. A projection of D. rosea growth rates by growthmodels without regard to water temperature showed much highergrowth potentials in the hypolimnion. However, with inclusionof water temperature, epilimnetic water always gave higher potentialgrowth rates. In this lake, warmer temperatures of epilimneticwater override the higher food quality and quantity providedby the deep-water chlorophyll maximum.     

Microzooplankton, vertical mixing and advection in a larval fish patch

A large ({small tilde}30 ? 75 km) patch of larval walleye pollock,Theragra chalcogramma, was located south of the Alaska Peninsuladuring May 1986. A drifter deployed in this patch followed ananticyclonic path consistent with dynamic topography. Changesin community composition and vertical distribution of microzooplankton>40 µm were sampled for 4 days alongside this drifterto examine feeding conditions for larvae. Biological and physicalchanges during the first 2 calm days revealed substantial small-scalevariability within the larger circulation pattern. Changes duringthe last 2 days were dominated by vertical mixing due to strongwinds. Despite mixing, prey concentrations remained adequatefor feeding by larval pollock as determined by laboratory studies.A satellite-tracked drifter replaced the first drifter and wasstill located within the patch 6 days later. Overall distributionsof larvae and movements of the drifters show a net translationof 7.8 km day^–1 south-westward, but details of the studyreveal complex interactions between coastal waters and a coastalcurrent. During the 10-day period there was an increase in standardlength of the larval fish population of 0.13 mm day^–1and a decline in abundance of {small tilde}7.6% day^–1.Both calculated rates must be underestimates due to continuingrecruitment of small larvae from hatching eggs.     

Landscape history in the subalpine karst region of Moncodeno (Lombardy Prealps, Northern Italy)

In situ larch stumps have been found above the present forest line in the karst region of Moncodeno, Grigna Settentrionale, northern Italy, an area where very few trees currently grow. Samples from living trees at the treeline and in the forest nearby were collected to develop a reference chronology for cross-dating the stumps. The latter span the period between 1218 and 1900, and together with the living trees, which cover the last 150 years, they have led to the development of a 784-year larch tree-ring chronology (1218–2001). Age and location of the stumps reflect past human activity. Deforestation and grazing have intensified the erosion processes in the area. The soil has thinned out and fragmented allowing karst landforms, previously buried by the soil, to emerge. Presently, the rocky outcrops limit the establishment of young larches so that only few scattered trees are currently found.     

The allelopathic effects of Festuca paniculata depend on competition in subalpine grasslands

Allelopathy is recognized as an important process in plant–plant interactions, but how it affects plant communities growing in competitive conditions has not been assessed. This article investigates whether the allelopathic effect of Festuca paniculata is modified by competition between target plants in subalpine grasslands. We hypothesized that plants growing in mixed stands will be more affected by allelochemicals than the same species in monoculture. At Lautaret pass (Northern French Alps), a pot experiment was designed. We used leachates from donor pots (Treatments: 1. Bare soil, 2. F. paniculata clipped, and 3. F. paniculata unclipped) to water target pots (Treatments: 1. Control (soil only), 2. Dactylis glomerata, 3. Agrostis capillaris, and 4. D. glomerata and A. capillaris). Target plants were cultivated during one growing season. The effects of leachates from donor pots and interspecific competition in target pots were evaluated by measuring the final biomass of plants. Soil fertility was controlled in all target pots by measuring NO₃ ^?, NH₄ ⁺, N, and C % of the soil. Effect of target treatment under bare soil : Both D. glomerata and A. capillaris grew better in monocultures than in mixture. Effect of donor treatment on monocultures : Under bare soil, D. glomerata grew better than under F. paniculata leachates. By contrast, A. capillaris did not respond to donor pot treatment. Effect of donor treatment on mixtures: However, when both species were cultivated together under F. paniculata leachates, the biomass of D. glomerata was similar to that in monoculture under bare soil. Differences in sensitivity to allelopathy reversed the impact of interspecific competition: A. capillaris facilitated D. glomerata under allelopathy, which made allelopathy of F. paniculata on D. glomerata inefficient. The complexity of overlapping mechanisms of plant–plant interactions are highlighted by this semi-natural experiment. In subalpine grasslands, allelopathy not only limits the growth of neighboring plants, but it may also modify community assembly by affecting other plant–plant interactions such as competition. This study contributes to explore the way allelopathy interacts with other plant–plant interactions in natural systems.     

Acid rain and its effects on sediments in lakes and streams

Wet and dry deposition of acidic substances, which are emitted to the atmosphere by human activities, have been falling on increasingly widespread areas throughout the world in recent decades. As a result, annual precipitation averages less than pH 4.5 over large areas of the Northern Temperate Zone, and not infrequently, individual rainstorms and cloud or fog-water events have pH values less than 3. Concurrently, thousands of lakes and streams in North America and Europe have become so acidified that they no longer support viable populations of fish and other organisms.Acid deposition may affect sediments in lakes and streams in a variety of ways. In particular, the sediment-water exchange of metals, sulfur, nitrogen and phosphorus, microbial processes, growth of periphyton and macrophytes, and benthic invertebrates may be affected.Overall, the effects of acid deposition on lake and stream ecosystems are the result of numerous and complex biogeochemical interactions, including catchment characteristics, flow path and residence time of water, and lake-basin morphometry and acid neutralization capacity of both aquatic and terrestrial (catchment) ecosystems.Suggestions for future research are given.