CONTENTS

1

Physical limnology of Italian lakes. 1. Relationship between morphometry and heat content
Walter AMBROSETTI and Luigi BARBANTI

The influence of morphometry on maximum and minimum heat content and on pluriannual heat budgets of 31 Italian lakes is analysed. The energy parameters are highly correlated with the morphometric parameters through power regressions. The lakes which markedly deviate from the regression curves are examined on the basis of their environmental conditions or their internal hy­drodynamics. In deep lakes (i.e. depth >50 m) there is a high correlation between depth and maximum and minimum heat content, while the heat budget is correlated only with fetch and depths of 50 and 90% annual heat exchange. In shallow lakes there is no re­lationship between fetch and heat budget, while it is high with all the considered depths. This is due to the different thickness of the water layer, in relation to the maximum depth, which is involved in the annual heat exchanges. We evaluated the water thicknesses within which 40 to 90% of the total heat exchanges take place; the results showed that in deep lakes 90% of the energy exchange is confined to a layer comprising 14-16% of the maximum depth, while in shallow lakes it can reach 82% of the maximum depth. In Lago Maggiore the 90% heat exchange depth increased by 0.34 m per year from 1963 to 1999, thus contributing to heating the whole water mass of the lake.

2

Physical limnology of Italian lakes. 2. Relationships between morphometric parameters, stability and Birgean work

Walter AMBROSETTI and Luigi BARBANTI

The relations of morphometric parameters with maximum stability (S) and maximum Birgean work (B) are analysed for 31 Italian lakes. Results show that in deep lakes the depths are more correlated with S, while fetch is more correlated with B. In con­trast, in shallow lakes the depths are correlated both with S and B, while the correlation with fetch is insignificant. Power regres­sions are used to define the relation between maximum depth, S and B, and an analysis is made of lakes showing a marked deviation from the curves, whether on account of their environmental features or their internal hydrodynamic state. In meromictic lakes total stability is subdivided into chemical and thermal stability; major inferences are made as to the depth of their winter mixing. The re­lationship between B and maximum depth is less significant than that between z_max and S, as lakes with depth >100 metres and therefore with a very thick hypolimnion do not fully satisfy the regression function. Lastly, heating efficiency (E) is evaluated and related to depth and surface area of the lakes; the conclusion is that shallow lakes are more efficiently heated than deep ones.

3

Zooplankton abundance and diversity in Lake Bracciano, Latium, Italy

Ornella FERRARA, Daria VAGAGGINI and Fiorenza G. MARGARITORA

The zooplankton community structure in Lake Bracciano (Latium, Central Italy) was studied in monthly surveys throughout an annual cycle (November 1998 – October 1999). The seasonal cycles and population dynamics of the dominant species are described and discussed. Copepods numerically dominated the community throughout the study period with calanoid Eudiaptomus padanus etruscosexsetosus making up the largest share of zooplankton density; moreover it accounted for the largest portion of total biovolume. Cladocerans represented a significant component of the zooplankton in the summer and autumn months. No substantial dif­ferences in regard to results of previous investigations (1971, 1972, 1984) were observed. The only differences for which there is evidence consist of the appearance of Filinia terminalis, never previously found in the lake, and the replacement of Keratella cochlearis instead of Kellicottia longispina as dominant species. However, the results of the comparison of the different investigations confirm that the trophic state of the pelagial region may be classified as oligo-mesotrophic.

4

Dynamics of bacteria and mixotrophic flagellates in an Alpine lake in relation to Daphnia population development

Cristiana CALLIERI, Roberto BERTONI and Gianluca CORNO

In a fishless oligotrophic Alpine lake (Lago Paione Superiore: LPS), with a simple trophic food web, the relationships between microorganisms and zooplankton have been studied. During the ice-free period 1999, bacterial grazing by mixotrophic flagellates (MxFl), autotrophic carbon fixation and excretion (EOC), bacterial production, and zooplankton abundance and biomass were measured weekly. After ice-melt, MxFl, which constituted 94% of total phytoplankton, increased in number until the appearance of Daphnia longispina, the dominant crustacean zooplankton species in LPS. Gross primary production and EOC ranged from 0.2 to 2.5 and from 0.1 to 1.8 µg C l^-1 h^-1, respectively. EOC accounted for 45 to 90% of total autotrophic production. Community grazing rate of MxFl was higher at 9 m in comparison with 1 m and ranged from 3 to 62 ´ 10³ bact ml^-1 h^-1. MxFl ingested 0.2 to 8% of the natural bacterioplankton per hour, in summer. The community grazing rate of mixotrophic flagellates was significantly correlated with the phytoplanktonic photosynthetic activity (P = 0.006) and with the bacterial production (P = 0.037). In August, Daphnia grazing caused a rapid decrease of both bacteria and MxFl and of their activities. Phytoplankton photosynthetic efficiency increased in late summer after a phosphorus pulse. Although P excretion by zooplankton was not measured, it is speculated that Daphnia play an important role in the availability of this element firstly by grazing bacteria and MxFl in the water column and, after the exhaus­tion of such food, by browsing algae at the water-sediment interface.

5

Comparing limnological changes associated with 19^th century canal construction and other catchment disturbances in four lakes within the Rideau Canal system, Ontario, Canada

Francine FORREST, Euan D. REAVIE and John P. SMOL

Paleolimnological analysis of microfossils and physical sediment characteristics in ²¹⁰Pb and Ambrosia dated sediment cores, along with diatom-inferred total phosphorus concentration [TP] reconstructions, were used to determine the trophic histories (ca 200 years) of four lakes within the Rideau Canal system, Ontario, Canada. Paleoecological information of the dominant diatom taxa that flourished during the pre-settlement period indicated that these lakes were naturally oligo-mesotrophic. At the estimated time of canal construction, all lakes demonstrated an increase in nutrients but their responses varied in magnitude. These differences were likely related to a number of variables, but the surface-area : watershed ratio appeared to be an important explanatory variable. Additionally, the similar trophic response of the control lake (not part of the canal), Otter Lake, illustrated the regional impact of past watershed disturbance (e.g. logging, settlement, mining, agriculture), not directly related to canal construction. In more recent years (~1970 to present), less productive planktonic species (e.g. Cyclotella comensis and Cyclotella aff. gordonensis) increased in all the study lakes. These recent water quality changes were attributed to improved nutrient retention of developing soils in secon­dary growth forests, the potential effects of climate warming, as well as mitigation efforts (e.g. decreased phosphorus concentrations in detergents, etc.). Eutrophication patterns determined for the deeper study lakes were similar to paleolimnological studies of other deep lakes in the canal system. However, the trophic response in the shallow lake, Lower Rideau Lake, is more pronounced at the time of canal construction than those of other shallow canal lake responses (e.g. nearby Lake Opinicon) and suggests that both alternative equilibrium states have occurred. This heightened response was attributed to increased nutrient export in Lower Rideau Lake’s limestone catchment and/or higher watershed disturbance. Finally, results from this study furthers our understanding of impacts in an integrated system of lakes and this information can be used to help set realistic mitigation targets for these and other lakes in the Rideau Canal system.

6

Looking beyond the shores of the United Kingdom: addenda for the application of River Habitat Survey in Southern European rivers

Andrea BUFFAGNI and Joanna L. KEMP

River Habitat Survey (RHS) is a system which records and quantifies the physical and vegetational structure of river channels and their immediate floodplains. In the United Kingdom, where it has been applied since the 1990s, it has brought an understanding of state of rivers nationally and has proved to be a useful part of scientific investigation. It is now obvious that such a method should be applied more widely, especially in the European context, where river data is lacking for many countries and there is a need for a standard, internationally comparable method. In this paper an extension to the basic survey method is presented, with the aim of im­proving the detail and quality of data collected for highly dynamic, braided rivers, more common in the rest of Europe, particularly the south, than in the UK itself. The changes to the survey form included the recording of secondary flow and substrate types for each transect, in addition to the usual recording of primary types. Where more than one wetted channel was present data were collected for both the main and secondary channels. These were common in the areas studied, for instance in autumn 2000 secondary channels were found at 9 out of 11 sites in northern Italy and 4 out of 11 sites in the south. Additionally, the results showed that with the re­cording of both primary and secondary flow types for each transect the average number of flow types found per site was increased by between 1 - 2.2. For substrate types the average increase per site was close to 1. Certain flow types, in particular ‘chute’ and ‘no perceptible’, and substrate types, for example ‘sand’, tended to be under-represented by the basic survey method. The relevance and implications of these results are discussed with respect to the southern European situation and the point is made that the detail re­quired from RHS depends on the original motivation for choosing to apply it. Where it is part of a detailed biological or ecological study the extra information provided by the extended form presented here is potentially useful.

7

Phosphorus, carbon and nitrogen enrichment during sedimentation in a seasonally anoxic lake (Lake Lugano, Switzerland)

Mauro L. Veronesi, Alberto Barbieri and Kurt W. Hanselmann

Sedimentation fluxes of major nutrients are investigated during 1996 and 1997 at three different depths and two locations in eutrophic southern basin of Lake Lugano (Switzerland). Horizontal differences between the two sites are on the order of 10-40% (but can exceed 50%), whereas differences related to interannual oscillations range between 5 and 24%. Particulate organic carbon (POC) and nitrogen (PN) fluxes show a constant increase of 5-20% from the upper to the bottom trap. This tendency remains more or less constant during the year. On the contrary, particulate phosphorus (PP) shows a seasonal variation, with higher accumulation rates from the I to the III trap in autumn and winter which can exceed +1200%. This phenomenon is due to the interaction between the dissolved phosphorus (DP) and the iron(oxi)hydroxides (Fe(OH)₃) near the oxycline. Fe(OH)₃ precipitates at the iron redox boundary, scavenging DP. This enrichment flux increases together with the development of the anoxic benthic layer. The efficiency of the iron redox layer in trapping upward diffusing P is related to the concentration of dissolved iron in the anoxic hypolimnion. In Lake Lugano the two considered sites present major difference of iron concentration, and this difference is reflected in the P sedi­mentation fluxes. The exposition of an additional sedimentation trap above the maximal oxycline height has allowed to gain insight into this phenomenon.

8

Zooplankton of 15 lakes in the Southern Central Alps: comparison of recent and past (pre-ca 1850 AD) communities

Marina MANCA and Michele ARMIRAGLIO

We report the results of a study on modern zooplankton community as well as on Cladocera sub-fossil remains from 15 high al­titude lakes in the Italian Central-Southern Alps, aimed at investigating changes after 19^th Century industrialisation. Present-day communities were compared to those of the pre-industrial period (approximately pre-1850 AD). Numerical analysis suggests a rela­tionship between environmental variables, such as pH or total phosphorus, and the cladoceran community structure. However, as found in previous studies, another important environmental variable not included in numerical analysis, is presence or absence of fish. Arctodiaptomus alpinus was not found at pH levels lower than 6.21, while Daphnia lacked from lakes where fish were regularly introduced. Daphnia re-colonised the today fish-less Lago Paione Inferiore, whereas in the past, with a regular introduction of fish, only rotifers and a few Cyclops were found. The re-colonisation is interpreted by regarding the three Paione lakes as parts of a cas­cade system, where a transport from the upper Lago Paione Superiore to the lower Lago Paione Inferiore can take place, and the sediments represent a kind of an "egg bank".