Journal of Limnology

(formerly Memorie dell'Istituto Italiano di Idrobiologia)

vol. 60(2), 2001

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CONTENTS
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J.L. NOTES

LAKE ORTA: A CASE STUDY (Part 2)

1- The trophic status of Bidighinzu Reservoir (Sardinia) before and after the diversion of waste waters
Antonella LUGLIÈ, Yelda AKTAN, Paola CASIDDU and Nicola SECHI

The purpose of this paper was to assess the effect of the diversion of wastewater on the trophic status of Lake Bidighinzu, a hy­pertrophic man-made lake in Northern Sardinia, used as a drinking water reservoir. There have been problems with potabilization since the early years of the diversion operation, particularly in the summer-autumn period. Data available (August 1978, February 1979 and March 1985) before the reservoir (1987) were compared with those collected during a study carried out in the annual cy­cle immediately after (1988-1989) and after some years (1994 and 1996-1997). The study examined the dynamics of temperature, main nutrients (total phosphorus, nitrate and ammonia), chlorophyll-a and phytoplanktonic biomass. No particular variation in the water nutrient availability emerged from the comparison between these two situations - especially for total phosphorus, whose an­nual mean concentrations were similar in the two annual cycles (386 mg P m^-3 in 1988-1989 and 305 mg P m^-3 in 1996-1997). Chlo­rophyll-a and biomass were high during each period of study (annual mean values were 17 mg m^-3 and 3.7 mg l^-1 in 1988-1989 and 11 mg m^-3 and 4.6 mg l^-1 in 1996-1997). However, peaks were never higher than values recorded in August 1978 (112 mg m^-3 and 133 mg l^-1) due to an extraordinary bloom of Ceratium hirundinella (O.F. Müller). Species composition of phytoplankton was typical of highly trophic conditions and was frequently characterised by the presence of Cyanophyceae and Bacillariophyceae. Results dem­onstrated that, ten years after construction of the by-pass, the lake had shown no improvement in its trophic status. This might depend on many factors, among which the most likely might be the non-use or partial use of the diversion system, which allows the sewage to continue to flow into the lake and keep its trophic status almost unchanged.

2- A new species of Mesocyclops (Copepoda, Cyclopoida, Cyclopidae) from Southeastern Mexico
Martha A. GUTIÉRREZ-AGUIRRE and Eduardo SUÁREZ-MORALES

The taxonomical status of several neotropical species of Mesocyclops is still uncertain. Furthermore, several new species with a restricted distributional range have been described recently in this region. From littoral zooplankton samples collected in the low­lands of the State of Tabasco, Southeastern Mexico, male and female specimens of a previously unknown species of Mesocyclops were found. This new species is described herein based on both sexes and following the upgraded standards set for the taxonomic study of this genus, including SEM analysis. The new species differs from its congeners by a combination of several characters: the unique shape of the antennular hyaline membrane, the ornamentation of both maxillule and maxilla, the ornamentation of the fourth swimming leg, the shape and structure of the genital receptacle, and the ornamentation of the furcal rami. It is most closely related to other neotropical species such as M. meridianus Kiefer, 1926 and M. pseudomeridianus Defaye & Dussart, 1989. This is the elev­enth species of Mesocyclops known from Mexico, and also the third new species described upon Mexican material. Due to the affin­ity of the Tabasco fauna with the Caribbean and the South American copepod faunas, the new species is expected to have a relatively wide geographical distribution in the neotropical region.

3- Species-level and community-level data analyses reveal spatial differences and temporal change in the crustacean zooplankton of a large Canadian lake (Lake Simcoe, Ontario)
Kenneth H. NICHOLLS and Claudiu TUDORANCEA

Consensus-building univariate and multivariate data analyses were used to identify patterns in space and time over seven years among 12 sampling stations in a 720 km² hardwater Canadian lake (Lake Simcoe, Ontario, Canada). There were 15 copepods and 26 cladoceran species identified in samples collected throughout the May-October periods of 1986-1992 from Lake Simcoe. Eleven crustacean zooplankters accounted for 88% of the total average density of all recorded species in the lake. Most of these (the main exceptions being Eubosmina coregoni and Daphnia pulicaria) are ubiquitous taxa with wide environmental tolerances. Multivariate analyses of these data identified spatial differences and a temporal trend in community composition. The use of Cao et al.’s (1997a) "CY-dissimilarity" measure combined with Ward's Linkage clustering algorithm and non-metric multidimensional scaling ordination resulted in several clearly defined groups of sampling units (SUs), which apparently were separated predominantly on the basis of variables related to space (sampling station) and time (year). The 7-year record suggested several lines of evidence for trends in community structure. Averaged over all stations, the yearly intra-station dissimilarities steadily increased 78% by 1992. The greatest rates of increase in yearly intra-station dissimilarity with 1986 baselines were found for the deeper sampling stations. These whole community-level trends were the net results of a many different patterns of change identified in species-level analyses. Such patterns included increasing densities over the 7-year period of Daphnia longiremis and Diaphanosoma birgei and declining densities of Daphnia pulicaria. Many other taxa showed highest values in the middle years of the study. Community-level temporal trends were least evident at sampling locations in the shallower and warmer Cook’s Bay, despite clear trends in several individual species. A significantly greater representation by large-bodied zooplankton species in the latter years of the study was consistent with declining stocks of rainbow smelt (Osmerus mordax) and lake herring (Coregonus artedii), suggesting the associated relaxation of predation pressure as a possible cause. Several species indicative of eutrophic states (Acanthocyclops vernalis, Chydorus sphaericus and Bos­mina longirostris) were common in the samples. The scarcity of the calanoid Senecella calanoides and the absence of another classic oligotrophic indicator calanoid, Limnocalanus macrurus in the Lake Simcoe samples suggests impairment of their deep, cold-water habitat and is consistent with the modelled historical loss of hypolimnetic dissolved oxygen in Lake Simcoe.

4- Phytoplankton and littoral epilithic diatoms in high mountain lakes of the Adamello-Brenta Regional Park (Trentino, Italy) and their relation to trophic status and acidification risk
Monica TOLOTTI

A survey of phytoplankton and littoral epilithic diatom communities was carried out on 16 high mountain lakes in the Adamello-Brenta Regional Park (NE Italy) as part of a wider research project aimed to the limnological characterisation of the seldom-studied lakes in this Alpine Region. The regional study was supplemented by the analysis of seasonal variations in two representative lakes. The principal goals of this paper are 1) to identify the most important environmental variables regulating patterns in the species composition of both phytoplankton and littoral diatoms, 2) to evaluate whether these algal communities can be used to improve tro­phic classification and 3) whether they can facilitate monitoring of diffuse human impacts (e.g. airborne pollution) on high altitude lakes. The relevance to monitoring is based on the acid sensitivity of all lakes studied, as indicated by the very low average alkalinity values (4-97 µeq l^-1) recorded during the investigation period. Chlorophyll-a concentrations and phytoplankton biovolume recorded in the lakes were very low, with maxima in the deep-water layers and in late summer. Phytoplankton communities were dominated by flagellated algae (Chrysophyceae and Dinophyceae). Several coccal green algae were present, while planktonic diatoms were almost completely absent. Littoral diatom communities were dominated by alpine and acidophilous taxa (mainly belonging to the genera Achnanthes and Eunotia). Trophic classification based on phytoplankton and littoral diatoms, respectively, ascribed all lakes to the oligotrophic range. In both algal communities species indicative of acidified conditions were found. Multivariate analyses indicated that both the regional distribution and seasonal variation of phytoplankton are mainly driven by nutrient concentration. Diatoms are predominantly affected by geochemical characteristics including pH and mineralization level.

5- Predatory impact of the mosquitofish (Gambusia holbrooki Girard) on zooplanktonic populations in a pond at Tenuta di Castelporziano (Rome, Central Italy)
Fiorenza G. MARGARITORA, Ornella FERRARA and Daria VAGAGGINI

A study of a permanent pond located in a nature reserve in Central Italy was carried out over two periods (1985-87; 1995-98) to determine the structure and dynamics of planktonic biocoenosis. The composition of the zooplankton community was quite different in the two periods of study: in the first period, rotifers, small-bodied cladocerans and larval copepods were the dominant groups; in the second there was a shift from rotifers and  microcrustaceans to dominance by large-bodied Daphnidae and adult copepods. The possibility is stressed that Gambusia holbrooki, present in 1985 and absent in 1995, may be responsible for the changes in the planktonic community.

6- Santa Giusta Lagoon (Sardinia): phytoplankton and nutrients before and after waste water diversion
Nicola SECHI, Fatima FIOCCA, Antonietta SANNIO and Antonella LUGLIÈ

The purpose of this paper was to assess the effect of the diversion of waste water on the trophic status of the Stagno di Santa Giusta, a hypertrophic lagoon on the centre-western coast of Sardinia. The data available (1990, 1992-1994) before the diversion (1995) were compared with those collected from 1995 to 1999. The lagoon has an area of about 8 km²; its mean depth is about 1 m and is well mixed as regards circulation and stratification. This study examined temperature, salinity, pH, main nutrients (reactive and total phosphorus, nitrate and ammonium nitrogen, reactive silica), chlorophyll-a, phytoplanktonic density and phytobenthic bio­mass. A comparison of the condition of the water before and after the diversion showed that there was an insufficient reduction in nutrient concentrations. The mean concentrations of nitrate nitrogen and ammonium nitrogen fell from 33 and 87 mg N m^-3, respec­tively, to 28 and 66 mg N m^-3, respectively. There was a more marked reduction in phosphorus (from 107 mg P m^-3 as reactive phos­phorus and from 190 mg P m^-3 as total phosphorus to 77 and 127 mg P m^-3, respectively), and in reactive silica, from 3 to 1.7 mg l^-1. Chlorophyll-a decreased from 11.3 to 10.2 mg m^-3; the total density of phytoplankton dropped considerably, but this was due to a different species composition. Phytobenthic biomass showed no particular variations. There were no changes in trophic level, so that dystrophic crises still occurred after the diversion. The results showed that there were no substantial improvements in the trophic status of the lagoon during the years after the diversion, except for some months in 1995. A possible explanation may lie in the structure of the diversion system, which does not completely stop the inflow of sewage into the lagoon.

7- Occurrence of high concentrations of a unique degradation product of chlorophyll-a in particles residing below the thermocline throughout a period of oxygen depletion in Lake Kinneret
Yosef Z. YACOBI, Ilia OSTROVSKY and Helen M. TALBOT

An unrecognized chlorophyllous pigment, designated Pheob4, was the most abundant degradation product of chlorophyll-a (Chl-a) in the hypolimimnion, benthic boundary layer and sedimenting particles in Lake Kinneret in June-July 1997, when a prolonged period of residual oxygen concentration was found below the thermocline. The highest concentration of Pheob4 was found in the benthic boundary layer, where it reached as high concentration as 2 µg l^-1. In sediment traps positioned below the thermocline Pheob4 constituted from about 5 to 20% of the Chl-a concentration and in bottom sediments from 5 to 185%. Pheob4 was not de­tected in epilimnetic water samples, but was found in sediment traps located within the oxygenated water column, within the range of surface wave action. Comparison of regular traps, with traps poisoned by formaldehyde indicates that the importance of Pheob4-containing particles in the sedimentation flux is relatively low, and that the bulk of Pheob4 found in the bottom sediment is appar­ently a result of Chl-a transformation in situ by biological activity. The timing and location of Pheob4 appearance coincided with the lake compartments where denitrification apparently took place. This suggests that Pheob4 may be used as a target pigment to trace organic matter processing under transient conditions in the water column and on the bottom sediments. Pheob4 was tentatively identified as 13², 17² cyclophaeophorbide-a enol. But, since the identification was done on a different system than the one used for the analysis of most of the samples, further investigation is required to verify that finding.

8- Branchiobdellid size-crayfish size: a possible relationship
Mario MORI, Yuri PRETONI, Sebastiano SALVIDIO and Andrea BALDUZZI

The aim of this paper was to verify a possible relationship between the size of the ectocommensal annelid Branchiobdella italica Canegallo, 1929 and that of its host, the freshwater crayfish Austropotamobius pallipes (Lereboullet 1858). 225 worms were col­lected from a crayfish population inhabiting a stream in central Liguria, north-western Italy. The sizes of B. italica ranged from 1 to 3.7 mm total length. The highest proportion of immature worms (41.7%) was collected from juvenile crayfish (size-range 11-18 mm carapace length), 15% from subadult (19-26 mm) and 3.4% from adult crayfish (³27 mm). The size-frequency distributions of worms from these three crayfish age groups differed significantly, except those from subadult and adult crayfish. It is arguable that the scarce presence of immature branchiobdellids on subadult and adult crayfish is due to the strong competition for space and food that these small worms  have to withstand from co-specific adults.

9- Chemical composition of Lake Orta sediments
Renato BAUDO and Monica BELTRAMI

Lake Orta (18.2 km², 1.3 km³, 143 m max. depth) has been severely polluted since industrialisation of its watershed began in 1926, at which time the lake began to receive industrial effluents containing high concentrations of copper and ammonia. Chro­mium‑, nickel-, and zinc-rich effluents from plating factories have also contributed to pollution levels, and pH -levels dropped below 4.0 as a result of the oxidation of ammonia to nitrates. More than 60 papers have documented the evolution of the chemical charac­teristics of both water and sediment, and the sudden decline of plankton, as well as benthos and fish. As a remedial action the lake was limed from May 1989 to June 1990 with 10,900 tons of CaCO₃. The treatment was immediately effective in raising the pH and decreasing the metal concentrations in the water column, and plankton and fish communities quickly rebounded. However, the chemical characteristics of sediments were influenced by the liming to a much lesser extent. Since 900 tons of copper and the same amount of chromium were contained in the top 10 cm of sediment, it appears likely that the sediment could potentially act as a cur­rent and future source of these metals to the water column. This observation has resulted in the implementation of a vigorous moni­toring regime to track the post-liming recovery of Lake Orta.

10- The littoral benthon community of Lake Orta after liming: a comparison between summer 1993 and summer 1998
Ettore BIELLI and Marina TESAURO

At different times in recent years (before, during and after liming) we have studied the littoral macrobenthonic community in Lake Orta, and, for comparison, in Lake Mergozzo (an unpolluted lake). In this paper we compare the situations after liming in summer 1993 and in summer 1998. We found no clear difference between the summer samples in 1993 and 1998 for each site; only seasonal fluctuations were in evidence, in particular in the sites of Gozzano and Pella and in Lake Mergozzo. The two lakes, how­ever, still present marked differences in the composition of their macrobentonic communities.

11- Benthos of Lake Orta in the year 1996
Renato BAUDO, Anna OCCHIPINTI, Anna Maria NOCENTINI and Monica SABOLLA

Lake Orta has been interested since 1926 by severe industrial pollution, due to the effluents of a rayon (artificial silk) factory that discharged huge amounts of copper and ammonia. As a consequence, the benthic fauna faced noticeable changes, becoming ex­tremely poor in species. However, after extensive liming (in 1989-90), the biotic communities of the lake showed many signs of re­covery, and in 1996 it was decided to study the benthos composition and abundance in detail, to assess whether the lake restoration resulted in a complete recovery  of this biotic compartment. Actually, both the small number of taxa, and the low abundance of the benthos, indicate that the sedimentary environment is still very stressed. In fact, albeit this study shows that benthos fauna is actively recolonising Lake Orta bottom, spreading from river mouths, in 1996 the benthic community cannot be considered fully organised.

12- Fourteen years of palaeolimnological research of a past industrial polluted lake (L. Orta, Northern Italy): an overview
Piero GUILIZZONI, Andrea LAMI, Aldo MARCHETTO, Peter G. APPLEBY and Francesca ALVISI

The first sediment core analyses were carried out in 1958, but it is only from 1985 onward that a modern palaeolimnological ap­proach was applied to the study of Lake Orta, a large and deep lake in North Italy, heavily polluted by ammonia and copper for about 50 and 30 years, respectively. Thus, we summarise those studies from a variety of sediment cores, collected in different years and sites, using both published and unpublished data. Changes in algal pigments (mainly chlorophyll derivatives, total and single carotenoids), inorganic geochemistry, especially heavy metals (e.g. Cu), diatoms, Cladocera, and thecamoebians were studied and related to the stressed environment. The whole picture reveals a close relationship between modifications in algal biomass, density, taxonomic composition and organisms size on the one hand, and water chemistry changes on the other hand. Long-term history of this lake over 7-8 centuries, including invertebrate and terrestrial vegetation dynamics (pollen stratigraphy), reveals close relation­ship with natural (climate) and anthropogenic forces.

13- Assessing sediment contamination using six toxicity assays
G. Allen BURTON Jr., Renato BAUDO, Monica BELTRAMI, and Carolyn ROWLAND

An evaluation of sediment toxicity at Lake Orta, Italy was conducted to compare a toxicity test battery of 6 assays and to evaluate the extent of sediment contamination at various sediment depths. Lake Orta received excessive loadings of copper and ammonia during the 1900’s until a large remediation effort was conducted in 1989-90 using lime addition. Since that time, the lake has shown signs of a steady recovery of biological communities. The study results showed acute toxicity still exists in sediments at a depth of 5 cm and greater. Assays that detected the highest levels of toxicity were two whole sediment exposures (7 d) using Hyalella azteca and Ceriodaphnia dubia. The Microtox^R assay using pore water was the third most sensitive assay. The Thamnotox, Rototox, Microtox solid phase, and Seed Germination-Root Elongation (pore and solid phase) assays showed occasional to no toxicity. Based on simi­larity of responses and assay sensitivity, the two most useful assays were the C. dubia (or H. azteca) and Microtox pore water. These assays were effective at describing sediment toxicity in a weight-of-evidence approach.

14- Assessment of Lake Orta sediments phytotoxicity after the liming treatment
Paola BARBERO, Monica BELTRAMI, Renato BAUDO and Daria ROSSI

The paper presents results from seed germination and root elongation tests for the evaluation of Lake Orta sediment contamina­tion.  The protocol for this test was developed after a literature survey and careful investigation of the proper conditions for obtain­ing the germination of commercially available seeds of several plants. Cucumis sativus, Lactuca sativa, and Lepidium sativum were finally chosen and used to detect the phytotoxicity of both liquid and solid phases of Lake Orta sediments.  To assess whether ex­change at the water – sediment interface could represent a hazard for the environment and the actual bioavailability of the toxicants present into the sediments, the phytotoxicity of samples collected in 1992, 1994, 1996, 1997, and 1998 from selected stations was tested on seeds of three different plant species (Lactuca sativa, Lepidium sativum and Cucumis sativus). The obtained results allow drawing distribution maps of the germination index, which confirm the persistence of toxic effects. However, the lake seems to posi­tively respond to the restoration treatment, since the more contaminated layers of sediments are presently buried under less toxic sediments.

15- In situ toxicity testing of Lake Orta sediments
Renato BAUDO, Daria ROSSI and Monica BELTRAMI

The use of in situ assays, which was initially proposed by Nebeker et al. (1984), is presently recognised as an effective tool to study the effects of contaminated sediments (Burton 1992). The placement of caged organisms to study short-term effects of exposure to contaminated environments has been employed for a number of aquatic species, both fish and invertebrate. This paper describes the application of this techniques to study the toxicity of Lake Orta sediments using both resident (Daphnia obtusa, D. longispina) and non-resident (D. magna, Echinogammarus stammeri) invertebrate species. In each of the selected stations, a group of four chambers, each containing 10 individuals, were deployed by scuba divers at 10 m depth and collected after 48 h. For each chamber, the surviving animals were counted, transferred to the laboratory and kept in Lake Orta water until their death. The number of neo­nates produced by each female in the laboratory was recorded daily in order to determine if the short exposure period could affect the reproductive behaviour of the animals. The technique and results reported here indicate the utility of in situ testing and suggest that, under certain conditions, this method of testing may yield results which are more representative of actual environmental condi­tions by avoiding the sample manipulation required for traditional laboratory toxicity tests.

16- Has Lake Orta completely recovered from its heavy polluted condition? A seventy years long history
Carla BONACINA

Taking in account the results obtained from the 16 previous papers, the present conditions of Lake Orta are stated. While chemi­cal features are now almost normal, it was demonstrated that sediments still contain large amounts of heavy metals in the upper 10 cm and are thus to be considered still toxic for benthic fauna. Algal community shows evidence of increasing complexity and stabil­ity, so does zooplankton, albeit to a lesser extent. Benthic fauna is still affected by past water pollution and is very poor both in num­ber of species and of individuals.