Convenor: NIVA


1.1 Introduction

The primary focus of the AL:PE project (Wathne et al. 1995) was to assess the extent to which remote mountain lakes in Europe were acidified as a result of "acid rain". The results (e.g. Battarbee et al. in prep., Mosello et al. 1995, Wathne et al. 1993, 1995 and in prep.) have shown:

- that many mountain lakes are acidified and that the extent of acidification depends on sensitivity (base cation concentration) and loading (acid deposition);

- that nitrate concentrations increase from north-west to south-east indicating the increasing role of atmospheric nitrogen deposition in the acid-base chemistry of surface waters in Europe;

- that diatom, zooplankton, invertebrate and fish populations found in these mountain lakes are very similar across Europe, but are often different from those in lakes with similar chemistry but lower elevation and less extreme climate.

We now propose to build on this knowledge to assess the impact of acid deposition on the dynamics of remote mountain lake ecosystems, and to develop models that relate acidic inputs to water chemistry, community structure and nutrient cycling.

1.2 Specific objectives

- to measure sulphur and nitrogen deposition at study sites to quantify acidifying inputs;

- to assess the seasonal variability of water chemistry at each site with respect especially to the dynamic behaviour of nitrogen, phosphorus and acidity;

- to assess the seasonal variability in populations of diatoms, zooplankton, littoral and profundal invertebrates in relation to seasonal changes in the physical and chemical environment;

- to test the hypothesis that histological and physiological analyses of fish populations can be used as early indicators of acid stress;

- to investigate whether the importance of microbial loops in the pelagic food web increases along the acidity gradient;

- to evaluate the applicability of empirical, steady-state, mass balance and dynamic critical load models to mountain lakes and to develop linked chemical-biological models for scenario assessment with respect to future EU and UNECE sulphur and nitrogen protocols.

1.3 Sites

12 sites sensitive to acidification located along north-south and west-east acid deposition gradients in Europe have been selected from the AL:PE dataset, or from comparative national studies, as shown in Figure 1. In addition, four 'secondary' sites (Arresjoen on Svalbard, Etang d'Aube in the French Pyrenees, Schwarzee ob Solden in the Austrian Alps and Lago di Latte in the Italian Tyrol) from AL:PE have been included for a sampling programme of reduced intensity to extend further the geographical coverage of this study, and to be used for model validation.

1.4 Project description and methods

1.4.1 On site measurements of sulphur and nitrogen deposition

Lead laboratories: NIVA, UCL, CNR

Acid deposition data used in the AL:PE project were derived from the nearest national monitoring station to each site, with values for individual sites being interpolated from model calculations. In most cases the monitoring stations were situated in lowland regions. Although the models can take some account of altitudinal differences (e.g. Dore et al. 1992), it is essential in this project that more accurate data for modelling are acquired through direct measurement.

Consequently, deposition will be measured over at least a 24 month period at each of the 12 primary lakes in this work package. The minimum requirement is for bi-weekly sampling of bulk-collected precipitation and deposition samples, which will be analysed for all major ions according to agreed protocols. Particular attention will be given to nitrogen compounds to assess better the role of nitrogen. Sampling and analysis is the responsibility of site operators. A full analytical quality control (AQC) programme has been instigated and run in conjunction with that for lake-water chemistry.

1.4.2 Seasonal variability of water chemistry

Lead laboratories: NIVA, CNR

The AL:PE database contains high quality water chemistry for 11 sites during the period 1990-1994 and for 17 sites between 1992-1994. However, water samples were collected only in the summer, once per year and did not always include a full assessment of the determinands ammonium or phosphorus. In this project it is essential to sample frequently throughout the study period as nutrient values especially vary significantly over the algal growing season. It is also important to measure pH and alkalinity during all seasons.

Water samples are taken from the surface at the outlet of each lake over at least a 24 month period: weekly in the ice-melt season, monthly or bi-weekly in summer and bi-monthly in winter. The AL:PE chemical AQC programme has been maintained and extended to better address nitrogen and phosphorus determinations.

1.4.3 Seasonal variability of biota

Lead laboratories: UiB(ZI), FSCU, UCL

In the AL:PE project biota were sampled at each site only on one occasion each year, enabling comparisons between sites to be made and allowing a rigorous taxonomic quality control system to be established. It is now essential to sample the key biological groups through the season to assess how changing physical and chemical conditions control populations and life-cycles, and thereby how such responses can be used as early-warning indicators.

Sampling and analysis of diatoms, zooplankton and invertebrates follows the AL:PE protocols. Sampling is undertaken by site operators on three occasions during the ice-free period for invertebrates, diatoms and zooplankton. Analyses is carried out by the laboratories of site operators, with programmes of taxonomic harmonisation coordinated by UCL (diatoms), FSCU (zooplankton) and UiB(ZI) (invertebrates).

1.4.4 Test the hypothesis that histological and physiological attributes of fish can be used to indicate early acid stress

Lead laboratories: UIBK, NIVA, CNRS

Communities of algae, zooplankton and invertebrates respond to a changing environment by competitive interaction between species. Conversely, indicator species can then be used to identify environmental change. In contrast, fish are relatively long-lived organisms and mountain lakes rarely contain more than one or two species. Nevertheless it is clear from AL:PE studies (e.g. Hofer et al. 1994, Wathne et al. in prep.) that early signs of environmental change may be inferred from studies of population structures and from an analysis of gills and other organs and tissues. Consequently, the populations from those study sites where fish are present are being monitored closely during the year, and an attempt will be made to separate the effects of acidity from those of trace metals.

The sampling and basic analytical methods follows AL:PE protocols and is carried out by the site operators. New analyses for physiology and histology include haematology (blood plasma ions, hematocrit, plasma proteins), gill histology (gill tissue thickening, apiptosis, chloride cell numbers and types, mucus cell numbers and status, mucus characteristics), surface tissue histology, liver histology (special emphasis on histopathology and pathophysiology as well as histochemistry) and ovary histology and metallothionein in liver and kidney. All samples are shipped frozen to UIBK and CNRS where analyses are centralised.

Multivariate analyses of datasets will be undertaken to assess the relationships between fish health and chemical and geographical variables.

1.4.5 Test the hypothesis that microbial activity in the pelagic food web increases with acidification

Lead laboratories: HBI-ASCR, FSCU, FBG, UGR.ES, UIBK

The activities above (1.4.3 and 1.4.4) are concerned with the dynamic response of individual biotic groups to acid deposition. It is also necessary to consider the interaction between groups of organisms within the food web. It has been shown that in some low-alkalinity mountain lakes the process of acidification is accompanied by oligotrophication (Vyhnalek et al. 1994) and the diversity of trophic levels decreases, resulting in an increasing significance of the microbial loop assemblage in the pelagic system (Straskrabova & Simek 1993). The top trophic level may be expected to change progressively from fish to crustacean plankton to microbes (protozoans). In acidified lakes the structure of the pelagic food web, especially the role of microbial activity, is the main determinant of the ecosystem function and carbon fluxes. Conversely, microbial activity may then be used as an indicator of acidification and ultimately of recovery.

No work of this kind was carried out in the AL:PE project. However, the AL:PE dataset is essential in providing baseline data. The lakes to be studied (Figure 1) have different pelagic food web structures when classified according to the highest trophic level. Some of them still have fish; the more acid ones are fishless; and microbial assemblages may prevail in those with the clearest water and lowest pH. Key processes within such simplified systems are carbon fluxes from phytoplankton to bacteria and protozoa, and feedbacks between these compartments.

The following compartments of the pelagic system in lakes and their interactions are being assessed: biomass of bacteria, phytoplankton, heterotrophic nanoflagellates (HNF), ciliates and metazoic zooplankton, total dissolved phosphorus, total inorganic carbon, dissolved organic carbon.

There are three sampling and analysis stages:

Sampling for the assessment of biomass in the pelagic food web

Preserved samples for the assessment of abundance and biomass are gathered by site operators at the 12 primary sites at routine sampling intervals of four weeks during the ice-free period. Samples are taken and processed according to stringent, agreed protocols. The abundance and biomass of bacteria, picocyanobacteria (Macjisaac & Stockner 1993) and HNF (Chrzanowski & Simek 1990) (1st level microbiology) is assessed after concentration on nuclepore filters and DAPI staining in epifluorescence microscopes and volumes measured by image analysis (Psenner 1993). Ciliates and phytoplankton are determined taxonomically and phytoplankton counted and volumes measured in sedimentation chambers in an inverted microscope.

Sampling and measurement of fluxes within the microbial loop

These specialised analyses are being undertaken at a sub-set of nine lakes (2nd level microbiology) by experienced partners. Primary production, exudation by phytoplankton and bacterial production (Riemann & Sondergaard 1986, Sell & Overbeck 1992) is measured by radionuclide techniques (14C and 3H) in the field and further elaborated in the laboratory by liquid scintillation measurement and a carbon analyser. Field samples are optimally taken over two-three periods of intense measurement during the ice-free period. Between two-four determinations are made for each measurement period at several water depths depending on the depth of the lake, on thermal stratification in the respective period and on the lake water transparency. Grazing rates of protozoans on picoplankton are estimated from the elimination (disappearance) rate (Salat & Marrase 1994) of fluorescently labelled bacteria (or algae) added to pre-filtered samples, using an epifluorescent microscope. These methods have been developed for lowland lakes (more trophic) and are being adapted to ultraoligotrophic mountain lakes.

Construction of a microbial "model"

Biomass (converted to carbon) will comprise the main input. The biomass of metazoic zooplankton and the composition of biological assemblages (fish, invertebrates and zooplankton) will be taken from the database generated elsewhere in this work package. The relevant data on lake water chemistry (nutrients, organic matter), transparency and temperature, inputs from the catchment and the atmosphere, and solar radiation will be incorporated from the work in 1.4.2 and work package 3. A seasonal model of carbon flux in the pelagic zone will be constructed according to the procedures of Stone & Berman (1993) and will be used to generate alternative flow chart scenarios for situations where the proportion of microbial processes vary according to the degree of acidification.

1.4.6 Evaluate applicability of various critical load models to mountain lake ecosystems, and develop a linked chemical-biological model for scenario assessment

Lead laboratories: NIVA, UCL

A range of lake acidification models, both steady-state and dynamic, have been developed in recent years to assess the response of surface waters to increasing or decreasing levels of acid deposition. At the moment it is not appropriate to apply these models to mountain lakes, principally because current sulphur and nitrogen deposition fields are not accurately specified and because the relationship between the structure and functioning of mountain lake communities and water chemistry is not fully established. However, the results of the sampling and analytical programme described here will allow the models to be fully calibrated.

The models to be used are:

- the diatom model (Battarbee 1995) - this model is an empirical model that sets the baseline critical load for a site;

- the steady-state water chemistry model (Henriksen et al. 1992) - this model requires water chemistry and catchment runoff data and is used to set critical load values for individual species, (e.g. arctic char), where a relationship between species and acid neutralising capacity has been established;

- the first order mass-balance model (FAB) (Posch et al. 1993) - this model requires a knowledge of catchment characteristics, especially nitrogen uptake and immobilisation in soils and vegetation. Unlike the empirical and steady-state models it takes into account the disequilibrium between nitrogen deposition and nitrogen leaching at sites where nitrogen is currently a limiting nutrient for catchment vegetation;

- the model of acid groundwaters in catchments - with aggregated nitrogen dynamics (MAGIC-WAND) (Jenkins et al. 1995) - this is a dynamic model, that requires the same information as the FAB model, but also includes rate coefficients allowing a time-dependent component to be included in predictions.

All models will be calibrated using data from all primary site catchments. For the FAB and MAGIC-WAND models additional detailed catchment specific information is required. Model validation will include the use of data from the AL:PE database. UCL is responsible for the diatom and FAB models, NIVA for the steady-state water chemistry and dynamic models.

The final stage in this work package will involve the linking of the dynamic chemical models with the biological models developed in 1.4.3, 1.4.4 and 1.4.5, to allow the biological consequences of alternative future acid deposition scenarios (e.g. according to UNECE protocols) to be assessed. In particular, a major aim will be an assessment of the extent and rate of recovery of acidified lakes with respect to species composition, fish health and food web dynamics.