CONTENTS
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J.L. NOTES

1

Metal concentrations in Unio pictorum mancus (Mollusca, Lamellibranchia) from of 12 Northern Italian lakes in relation to their trophic level

Oscar RAVERA, Gian Maria BEONE, Roberto CENCI and Paolo LODIGIANI

This research aims to test the reliability of environmental monitoring by bioaccumulators of pollutants; that is to establish a positive relationship between the pollutant concentrations in the bioaccumulator and those in the water in which it lives. To this end we analysed the contents of Al, Cu, Zn, Fe, Mn and Ca in the soft tissues and shell of Unio pictorum mancus. The filtered water sam­ples from the mussel habitat were analysed for the same metals. The mussels were collected from 15 stations settled in 12 Northern Italian lakes during the first two weeks of July 2001. These results were obtained: a) metal concentrations varied widely with mussel size and among stations; b) a significant positive correlation between the concentration of calcium in the water and in the mussel tis­sues, but no relationship emerged for the other metals; c) no relationship between the metal concentrations in the tissues and those in the shell was found; d) there was a certain tendency for Mn, Fe and Zn concentrations in the soft tissues to increase with shell size; e) the sequence of the decreasing metal concentrations arranged for the tissues was similar to that of the shell, but rather different from that in the water; and f) the concentration factor values of the trace metals were high for the shell and soft tissues. In highly productive lakes large size mussels dominated, whereas small mussels were more abundant in low productive lakes. Although the metal concentrations in the water of productive lakes were greater than in low productive ones, the metal concentrations in the tis­sues of the mussels from the latter were generally higher than those in the mussels from the former. We propose some hypotheses to explain this paradox. Finally, our results show that the metal concentrations in the mussels do not reflect the metal concentrations in the water in which they live. It follows that this commonly used but oversimplified monitoring system cannot be recommended. On the other hand mussels may be very useful for other purposes, such as identifying new pollutants or pollutants present in such low con­centrations that they cannot be measured with the commonly used methods. The pollutant content of mussels may enable the varia­tions in time of the pollutant level of an environment to be monitored. In addition, the transplantation of mussels from a clean site to a polluted one may be a useful tool for identify the pollutants of the receiving environment.

2

Algal composition and abundance in the neuston surface micro layer from a lake and pond in Virginia (U.S.A.)

Lubomira BURCHARDT and Harold G. MARSHALL

A comparative study was conducted that characterized the algae within the neuston, ca 2 mm below the surface, and the algae in the water column from two freshwater habitats. There were significant differences in total algal abundance and the abundance of diatoms, cyanoprokaryotes, and chlorophytes between the neuston and water column algae of these two regions during each season and at both sites. The pond neuston was dominated by chlorophytes, with total algal abundance ranging seasonally from 0.6 to 59.6 ´ 10^-3 cells ml^-1 compared to water column algal concentrations of 4.1 to 40.4 ´ 10^-3 cells ml^-1. The lake was dystrophic, with dia­toms the most common and abundant species, with the neuston algal abundance ranging from 0.09 to 1.31 ´ 10^-3 cells ml^-1, and the water column algae from 0.19 to 2.70 ´ 10^-3 cells ml^-1. Proximity to the variable nature of the surface layer was not a deterrent for neuston algal development, which frequently reached bloom status and contained a diverse assemblage of taxa.

3

Long-term changes of the crustacean zooplankton community in Lake Mjøsa, the largest lake in Norway

Jarl Eivind LØVIK and Gösta KJELLBERG

Lake Mjøsa has been subject to an accelerating eutrophication from the 1950s to the mid 1970s, but comprehensive nutrient re­duction efforts caused marked reductions of phytoplankton production and biomass during the 1980s, a process that continued dur­ing the 1990s. Zooplankton biomass and species composition was considerably affected during the eutrophication and subsequent oligotrophication. Total crustacean zooplankton biomass decreased along with decreasing algal biomass during the 1980s and 1990s. The seasonal means of zooplankton biomass were positively correlated with seasonal means of phytoplankton biovolume and chlorophyll-a, indicating a primarily bottom up regulation of the zooplankton biomass. Several herbivorous and omnivorous zoo­plankton species (Daphnia galeata, Bosmina longispina, Limnocalanus macrurus and Cyclops lacustris) were probably negatively affected by reduced algal biomass, whereas other species (Holopedium gibberum and Thermocyclops oithonoides/Mesocyclops leuckarti) seemed to be positively affected. H. gibberum disappeared in the 1960s, but reappeared in the 1980s after the significant reduction in algal biomass and primary production. The temporal trend of T. oithonoides/M. leuckarti indicated a strong competition with cladocerans (mainly B. longispina and D. galeata) in periods with high algal biomass. Early warming of the lake could also have promoted a biomass increase of T. oithonoides/M. leuckarti in later years, although the mean epilimnion temperature did not correlate with seasonal mean biomass of these species. The seasonal mean biomass of Eudiaptomus gracilis, the dominant calanoid, showed substantial fluctuations with 6-7 years between tops, but a decreasing trend during the 1990s. However, there were no sig­nificant correlations between this species and any of the environmental variables. The study indicated that dominant cladocerans (D. galeata and B. longispina) are decisive for the success of cisco (Coregonus albula), one of the most important planktivorous fish spe­cies, as also described by earlier studies. However it also demonstrated top down control on cladocerans in years with especially large stocks of cisco.

4

Characterisation and quantification of phosphorus release from profundal bottom sediments in two dimictic lakes during summer stratification

Uwe SELIG and Günter SCHLUNGBAUM

Phosphorus release from the sediment in two dimictic lakes was compared during the summer stratification. Soluble reactive phosphate (SRP) and also NH₄⁺ were released from the anoxic sediment into the water column in Lake Dudinghausen and Lake Tiefer. The sediment released NH₄⁺ and SRP only when oxygen and nitrate/nitrite were absent. Besides the SRP concentration in the hypolimnion, P-binding in the upper sediment changed during summer stratification in both lakes. In Lake Dudinghausen a decrease in the redox-sensitive bound P was observed in the upper sediment. Only in this lake does the classic anoxic P-release by reduction of the FeOOH-phosphate complex play a dominant role. In both lakes, however, the contents of organically bound P increased in the upper 0.5 cm layer during the summer stratification. P-release was calculated using four different methods. The calculated P-release during summer stratification varied between 15 and 207 mg P m^-2 y^-1 in Lake Dudinghausen and between 22 and 55 mg P m^-2 y^-1 in Lake Tiefer.

5

Do distinct water chemistry, reservoir age and disturbance make any difference on phosphatase activity?

Ana Maria GERALDES and Maria-José BOAVIDA

Alkaline phosphatase activity was assessed concomitantly with total phosphorus, orthophosphate and phosphomonoester con­centrations in two meso-eutrophic reservoirs with distinct age and subjected to different kinds of environmental influence. Differ­ences in conductivity, temperature and pH were found. However, during the study period alkaline phosphatase activity was similar in both reservoirs. Water colour was higher in S. Serrada Reservoir. This fact can be related to (1) reservoir age (2) high internal dis­turbance (3) large imputs of allochthonous detritus, resulting from the combined effect of grazing, fire and catchment slope. Despite the high water colour recorded in S. Serrada, alkaline phosphatase activity was apparently not inactivated by humic substances. Be­sides, the obtained results demonstrated that hydrolysis of phosphomonoesters by alkaline phosphatase was not important for ortho­phosphate regeneration in these reservoirs. Probably orthophosphate was always available to biota. In fact, in the experiments based on Selenastrum capricornutum Printz algal test, similar phytoplankton growth responses were obtained for different phosphorus concentrations. Thus, these results seem to indicate that phosphorus was not a limiting nutrient in either reservoir. Although phos­phatase activity was significantly correlated with some phytoplankton genera in both reservoirs, no significant correlations were found between enzyme activity and chlorophyll-a. Significant correlations between phosphatase activity and crustacean zooplankton were only recorded in S. Serrada. In spite of these results there was some indication that the main source of phosphatase might have been bacteria involved in decomposition processes instead of phyto- and zooplankton taxa.

 6

Recognizing toxic species in aquatic habitats: a potential concern in lake management

Mikołaj KOKOCINSKI and Harold G. MARSHALL

The importance of distinguishing toxic and non-toxic algal species is becoming a more common problem for management deci­sions associated with various freshwater and estuarine habitats. An example is given where two dinoflagellates, originally unidenti­fied as closely resembling the toxin producing Pfiesteria spp., have been compared to these species. In order to clarify any relation­ship to Pfiesteria spp., scanning electron microscopy was used to determine the plate tabulation patterns of these dinoflagellates and make comparisons to the tabulation present in Pfiesteria spp. The results indicated significant differences in the plate tabulations of these taxa to distinguish them from Pfiesteria spp.

7

Phytoplankton association patterns in the deep southern subalpine lakes

(Part 2)
G. Morabito, N. Salmaso & D. Ruggiu
(Guest Editors)

The three following papers constitute the second, and final, part of a series of papers dedicated to the phytoplankton of the deep subalpine Italian lakes (DSL). The first part, comprising an introduction and three papers on lakes Garda, Como and Maggiore respectively, was published in the volume 61 (1) of this journal (J. Limnol., 61). The research, carried out for three years (two years in L.Como) in the period 1997-2000, was a concerted effort by investigators of five Insitutes in Italy and Switzerland. It was generated by the awareness that, despite the large number of papers existing on the phytoplankton of the single DSL, those of a comprehensive nature are very few, and by the perceived interest of a comparative investigation on the phytoplankton of the large lakes in the Insubrian district in the light of the recent progress on the ecology of the freshwater algae. So, in the final paper following those on lakes Iseo and Lugano an effort is made to compare and characterize the different species assemblages in the background of geographical and morphological characteristics, and of recent changes in trophic gradients. Despite often important differences in these respects, and observed various community responses, a common pool of species seems to be identifiable in the district.

 Giuseppe Morabito, Nico Salmaso, Delio Ruggiu

8

Studies on the phytoplankton of the deep subalpine Lake Iseo

Letizia GARIBALDI, Antonella ANZANI, Alessandro MARIENI, Barbara LEONI and Rosario MOSELLO

This paper reports the results of investigations carried out on the chemical characteristics and phytoplankton community of Lake Iseo. Samplings were performed on a monthly basis from 1998 to 2000. At least three main algal groups dominated the community throughout the study period. The large Bacillariophyceae were dominant mainly during late winter and early spring (Aulacoseira spp., Melosira varians, Asterionella formosa), with few species able to maintain occasional positive growth also during mid summer and/or autumn (Fragilaria crotonensis and Diatoma elongatum). The thermal stability of the water column and silica depletion were the main factors responsible for the decline of the large spring diatoms. The subsequent growth of Mougeotia sp. (Conjugatophy­ceae) was favoured by its lower sinking rate and resistance to increasing grazing pressure by the dominant copepods (Copido­diaptomus steueri) and cladocerans (Daphnia hyalina ´ galeata). Among the cyanobacteria, the greater development of Planktothrix rubescens in the autumn months, with conditions of vertical homogenisation and decreasing Z_eu/Z_mix ratios, was favoured by its abil­ity to survive at low light irradiances. The temporal replacement of these three groups constitutes the main sequence of the annual phytoplankton succession in Lake Iseo. A large development of other algal groups was recorded only in one or two of the three study years (e.g. Dinophyceae and Chlorococcales). The changes observed in the annual phytoplankton development are discussed in the light of differences in the spring fertilisation of the waters, caused by differences in the depth of the layer involved in the late winter and spring vertical mixing.

9

Winter and spring mixing depths affect the trophic status and composition of phytoplankton in the northern meromictic basin of Lake Lugano

Marco SIMONA

The trophic state of Lake Lugano is still too high to be acceptable, despite extensive recovery measures undertaken in recent decades which have resulted in a reduction of the external phosphorus load to the deepest of the lake's basins (northern basin; Z_max=286 m) to fairly acceptable values. Since meromixis was established in the middle of last century, the deep hypolimnion of the northern basin (the layer between ca 100 m and the bottom) has contained high quantities of nutrients (especially phosphorus) which are a major potential source of internal load. When there are particularly strong winter mixing events, a portion of this phosphorus reserve is redistributed along the upper water column (0-100 m). The impact of meteo-climatic conditions on the plankton biocenosis were analysed using data collected in the northern basin (Gandria station) during the three-year period 1998-2000. The phyto­plankton composition, which is typical of eutrophicated waters, shows marked interannual variations, also depending on the degree of mixing of the waters at the start of the vegetative period. Though there is no steady pattern of typical dominant species / master species in the lake, there is a seasonal succession characterised by a marked development of diatoms in spring, and a predominance of chlorophyceans and cyanobacteria in summer and autumn. Under present conditions, the mechanisms of internal replenishment of nutrients towards the euphotic layer, due to the phenomena of late winter and spring mixing, constitute a significant source of nutri­ents for the spring and summer growth of phytoplankton. On the other hand, pronounced mixing phenomena, like those occurring in the two-year period 1999-2000, can reduce the hypolimnetic nutrient reserves and cause a decrease in the trophic potential of the basin, contrasting with an increase in algal biomass in the euphotic zone.

10

A synoptic study of phytoplankton in the deep lakes south of the Alps (lakes Garda, Iseo, Como, Lugano and Maggiore)

Nico SALMASO, Giuseppe MORABITO, Rosario MOSELLO, Letizia GARIBALDI, Marco SIMONA, Fabio BUZZI and Delio RUGGIU

This paper presents a synoptic account of the most important results emerging from studies on the phytoplankton communities in the deep southern subalpine lakes Garda, Iseo, Como, Lugano and Maggiore (DSL) in the second half of the 1990s. At present, the trophogenic layers of these lakes are trophically different, ranging from the oligo-mesotrophy of lakes Maggiore and Garda to the meso-eutrophy of lakes Iseo and Lugano. The research confirmed the existence of a common pool of species developing in the DSL, as already suggested by early studies conducted on a seasonal basis from the end of the 1970s to the first half of the 1980s. However, multivariate analyses (Correspondence Analysis, CA, and a subsequent application of Non Metric Multidimensional Scaling) demon­strated that the species in this common pool were developing differently or exclusively along a geographic and a trophic gradient. The major differences in the geographic distribution were found between the easternmost lakes (Garda and Iseo) and those farthest to the West (Lugano and, partly, Maggiore), with intermediate characteristics in Lake Como. These differences were due mainly to changes in the dominance relationships and only secondarily to compositional changes. The detection of the ultimate causes of these differences should take into account other factors not considered in the paper (i.e. the specific analysis of the food webs, local climatic conditions, hydrology and seasonal input of nutrients). Despite the observed differences, common patterns in the sequence of seasonal assemblages in the DSL could be recognised and defined. The second gradient in the species distribution identified by CA was strongly correlated with the principal trophic descriptors (algal biomass and total phosphorus); this meant that the phytoplankton taxa could be ranked along a trophic spectrum, from oligotrophy to eutrophy. A brief examination of the main differ­ences which have historically arisen with the progress of eutrophication in the DSL showed that the species identified in this study as being indicative of more eutrophic conditions were increasing in importance in some of the lakes.