Assessing Hypolimnetic Stress in a Monomictic ...

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Assessing Hypolimnetic Stress in a Monomictic, Eutrophic Lake Using Profundal Sediment and Macrobenthic Characteristics a

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Marcello Bazzanti , Marco Seminara , Sabrina Baldoni & M. Giuseppina Dowgiallo

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Department of Animal and Human Biology, University of Rome “La Sapienza”, V.le dell'Università 32, 00185, Rome, Italy b

Department of Plant Biology, University of Rome “La Sapienza”, P.le A. Moro 5, 00185, Rome, Italy Version of record first published: 06 Jan 2011.

To cite this article: Marcello Bazzanti, Marco Seminara, Sabrina Baldoni & M. Giuseppina Dowgiallo (1998): Assessing Hypolimnetic Stress in a Monomictic, Eutrophic Lake Using Profundal Sediment and Macrobenthic Characteristics, Journal of Freshwater Ecology, 13:4, 405-412 To link to this article: http://dx.doi.org/10.1080/02705060.1998.9663637

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Assessing Hypolimnetic Stress in a Monomictic, Eutrophic Lake Using Profundal Sediment and Macrobenthic Characteristics Marcello Bazzanti, Marco Seminara, and Sabrina Baldoni Department of Animal and Human Biology University of Rome "La Sapienza" V.le dell'Universit:l32, 00 185 Rome, ltaly

and M. Giuseppina Dowgiallo

Downloaded by [Universita Studi la Sapienza] at 06:31 20 November 2012

Department of Plant Biology University of Rome "La Sapienza" P.le A. Moro 5, 00185 Rome, ltaly

ABSTRACT Summer-winter variations of the physico-chemical features of profundal sediments and of the characteristics of the profundal benthic community in a small, monomictic and eutrophic lake were analyzed in order to define their relations to environmental stress in the hypolimnion. The sediments were rich in P and N compounds. The benthic fauna exhibited a very poor taxa richness and diversity in summer, probably due to reduced oxygen in the hypolimnion, and there was only slight improvement of these community parameters in winter when reoxygenation occurred. The macrofauna was influenced by both sediments and overlying waters.

INTRODUCTION Monomictic, eutrophic lakes of the Mediterranean region usually exhibit distinct features in summer thermo-chemical stratification and in winter mixing period (Hutchinson 1957). Summer stratification, often lasting until the autumn or later, can lead to anoxia and to production of hydrogen sulphide and ammonia over a wide area of the hypolimnion. In a recent paper (StraSkraba, 1996), the importance of hypolimnetic anoxia was stressed as one of most serious problems related to water quality in lake and reservoir management. Especially in anoxic conditions. the role played by sediment becomes very important for the nutrient budget of the entire lake because the sediment acts as both a reservoir and a source of contaminants for the water column (Chapman 1989). Winter mixing tends partially to improve the environmental conditions in the profundal zone through reoxygenation of the water. Long-lasting anoxia negatively affects growth and production of benthic invertebrates (Jonasson 1984) and their reproduction rate (Aston 1973) and increases faunal similarity among depths (Bazzanti and Seminara 1987). The diversity of the profundal benthos of eutrophic lakes is generally low and decreases in summer. being essentially dependent on the degree of environmental stress in the hypolimnion. This phenomenon is very pronounced in small, deep and severely eutrophic lakes or in meromictic lakes. where a lack of recovery of the prohndal benthic community may be expected from anoxic summer to improved winter conditions (Bazzanti and Seminara 1985, 1995).

Journal of Freshwater Ecology. Volume 13, Number 4 - December 1998

We report here the results of investigating the summer and winter physicochemical characteristics of profundal sediments and zoobenthos of Lake Percile (Central Ira]! ), which is undergoing a clear-cut eutrophication process.


STUDY AREA Lake Percile, situated in the Regional Park of the Lucretili Mountains, is a small. karstic lake (altitude: 720 m a.s.1.. surface area: 9000 m?. maximum depth: 15 m). which is strongly influenced by cattle browsing and agricultural activities in the surroundings. During the year of the study (March 1993-February 1994), annual mean values of the water transparency and of total phosphorus content were 3.7 m and 79 pgll. respectively. The hypolimnetic waters showed a marked deoxygenation in summer-autumn (lowest values: about 3 mgll at 10 m and 1 mgll at 14 m), with hydrogen sulphide present near the sediments and carbonate concentration increasing with depth (Tolomeo, unpublished data). According to the OECD (1982) classification. the lake can be considered as eutrophic, with the sole exception of the transparency. which still signals a mesotrophic condition. Moreover. in February 1994. the lake was affected by an Asterionella formosa bloom. The lake stratified thermally from late May to October and the profundal zone was defined as about > 8 in according to the thermal profile (Margaritora and Pappacoda 1996).

METHODS The sediments were collected using a standard Ekman grab (collecting area: 225 cm2)in summer (August, 1993) and winter (February, 1994) at eight stations located at two depths (10 and 15 m). A total of 48 biological and 16 physicochemical samples (three and one samples, respectively, for each station and season) were collected. The sediments were dried and analyzed for: CaCO, by the volumetric calcimeter method (Allison and Moodie 1965); total organic C by the Walkey-Black method (Gaudette et al. 1974); total P (Marengo and Baudo 1988): total N (TKN) by the Kieldahl method (Bremner 1965); organic matter (O.M.%) by loss of ignition at 450" C (Cummins 1962); SO; by colorimetric determination after precipitation of sulphates (SISS 1985); and granulometric composition, according to Cummins (1962). Benthic samples were filtered with a 0.28 mm mesh and preserved in 10% formalin. After log (x+l) or arcsine dp transformation of absolute and relative data, respectively (Sokal and Rohlf 1973), Principal Component Analysis (PCA) was performed to determine similarity among stations and to define environmental gradients. Spearman's coefficient (r,) was used to test for correlation among variables, and Mann-Withney's non parametric U-test was adopted to test differences in sediment and community parameters between the two seasons (Elliott 1977). Only correlations with a significance of at least p < 0.05 are cited in the text.


RESULTS The sediments of Lake Percile (Table 1) were moderately enriched by both total P at 15 m and N at 10 m; they were heavily enriched by total N at 15 m. The concentrations of C. N. P. SO,; and the content of organic matter, silt and clay correlated significantly and all increased with depth, whereas CaCO, and fine and very fine sand significantly decreased with depth. The PCA biplot (Figure 1) clearly distinguished the stations at the two sampling depths according to their different nutrient contents and granulometric composition. Moreover, the stations were also distinguished by season. with lower nutrient contents in the sediments during summer. The benthic fauna was generally characterized by low number of taxa and diversity, both decreasing significantly with increasing depth (Table 2). In summer the values of these community parameters were lower than in winter only at 10 m depth. Oligochaetes and chironomids showed a significant reduction of both density and biomass with increasing depth, P, N, C, sulphates and organic matter contents in the sediments. whereas chaoborids exhibited an opposite trend (Table 3). PCA performed on benthic data showed (Figure 2) a clear separation along the first axis between the station at 10 m in winter and all the other stations. The environmental characteristics of the sediments which showed significant correlations with PC 1 scores are indicated by arrows in Figure 2. Therefore, the first axis can be interpreted as a gradient of physico-chemical parameters of sediments and increasing anoxia of overlying waters in summer, which increased the faunal similarity of the stations located at the two different sampling depths. Also diversity correlated with this axis. The U-test showed a significant difference between the two seasons only for chironomids @ < 0.05). Chironomids had low density and biomass at both depths during the summer, but they showed significant increases at 10 m depth in the winter (Table 3). On the whole, the fauna at 15 m displayed practically no changes from summer to winter. Table 1. Mean values of the sediment variables of the stations at the same depth.

Summer

Winter

Depth Organic matter (%) Organic C (%) total N (%) total P (%) CaC03 (%) SO^= (meq1100g)

10 m 7.3 4.24 0.2 1 0.033 62.4 0.57

15 m 10.87 5.76 0.4 1 0.054 52.8 1.23

10 m 7.1 4.1 0.26 0.038 58.9 0.15

15 m 11.2 6.5 0.47 0.068 46.0 0.8 1

Sand (%) Silt (%) Clay (%)

45.9 45.1 9.0

25.4 60.6 14.0

45.0 49.0 6.0

14.8 75.7 9.5


Figure 1. Principal Component biplot of stations and environmental variables of the sediments. DISCUSSION The profundal macrobenthos of Lake Percile was composed mainly of Tobrilus gracilis, Limnodrilus hofmeisteri, Chironomus plumosus gr.,known to be tolerant of eutrophic conditions and deoxygenation (Schiemer and Duncan 1974, Saether 1979, Wiederholm 1980, Milbrink 1983) and of Chaoborusflavicans, which is extremely tolerant to severe anoxia and high hydrogen sulphide concentrations (ProkeSova 1963, Sikorowa, 1968). The remaining few taxa generally spread over a wide range of ecological requirements. Only Peloscolex velutinus, occurring at 10 m in winter with a very low density, is an oligotrophic species (Lang 1989). The diversity was low both in summer and in winter compared with lakes and reservoirs with a lower trophic degree (i.e. Ransom and Dorris 1972, Wisniewski and Dusoge 1983, 1986). Therefore, the composition and the structure of the profindal benthos accurately reflects the pollution of the sediments and the eutrophic conditions of the waters. Chironomids have a strong capability to colonize large areas by planktonic first-instar dispersal and by migrating older larvae, and they are known to be rapid colonizers of the sediments when the environmental conditions of lakes become less adverse (Wiederholm 1980, Lang and Reymond, 1996). Therefore, in Lake Percile we would have expected an appreciable winter increase of taxa richness and abundance of this dipteran family at both 10 and 15 m. In fact, the winter colonization of profundal sediments by chironomids seems to be quite slight and recognizable by a small increase of their density and biomass only at 10 m. An almost complete absence of winter recolonization of these dipterans has been documented in other profundal Italian lakes which undergo strong perturbation in the hypolimnion due to a severe eutrophy (Bazzanti and Seminara 1985) or to

meromisis (Bazzanti and Seminara 1995). Con\'ersely. in shallow eutrophic ~ a t e r s . several authors (Nocentini et al. 1974. Ferraris and Wilhm 1977. Beattie 1981. Bazzan~let al. 1989) recorded a decrease in the abundance of profundal benthos. especiall!, chironomids. in the summer-early autumn stratification and a subsequent appreciable increase of density and biomass values under improved winter conditions. In these waters. where anosic conditions can be relatively short-lasting. the macrobenthic community probably reacts differently than in the small. deeper lakes. which may be considered more sensitive to environmental degradation in the hypolimnion.


Table 2. Composition and Shannon diversity (range) of the profundal macrobenthic community according to the sampling depth and season.

Summer Depth

Winter

10m

15 m

10m

14

6

21

I)

OY1l-201

0-0-1 I S

? Oi-2 15

0 5 s - I \)1

ljm

Taxa Nernatoda Tobrrltts gracrlis (Bastian) Trrp.vla glomrrans Bastian Dotylarmus sragnalrs Dujardin

Oligochaeta ~Varssp. Limnodrrlus hofiersreri Claparede L. claparedeianus Ratzel immature Limnodrllus Pr/osco/ex velurinus (Gmbe) Psamrno~cridesbarbatus (Grube) Tub& rrrb,fk (Muller)

Ephemeroptera Caenidae Herniprera Corixidae Diptera Chironom~dae Psrcrroclarfi~cs.sor~iidrNr~s gr. Liibrundrnrir Pro~fadrus Tunypus kroacr (Kieffer) Pararanyrursus Cludoran~rorsusntancrrs gr. Tunyrarszts Pmrupedilurn Pol~pedilrmrnubeculosum yr G!lprormdtpes Dicrurendipes rrrrvosus gr. Gyprochrro~tom~rs Cladopelmu iuccophrla gr. Pilruchrrut7omtts ilrctrurrrs gr Ch~runumrrsplrrntosrrs gr.

D~prrraChaoboridar Chuohortts f1u~rc.mi.sPvlsigrn

Toral number of t a u

Sliannon divrrs~t! (14)

.4s alread!, h~rpothesizedfor other small and deep Italian lakes (Bazzanti and Seminara 1985, 1996). we suggest the follo~vingregarding Lake Percile: - the profundal benthos is strongly influenced not only by sediments but also by o\,erl!;ing Lvaters: and - deep or relatively deep. small. eutrophic lakes with severe deoxygenation in the h\,polimnion during several months of the year exhibit a strong summer simplification of benthic community (low taxonomic richness and diversity) which cannot. or cannot significantly. improve during the winter. depending upon the extent of the stress. --

-.


Clay

--

-

O.M.%

P N

-

-

-

Sulphales

C

0

CF

Sand

0

cacg

TaxonmGcdivmity

b

Figure 2. Principal Component Analysis of biological parameters (density of taxa). See text for further explanations. Table 3. Mean values of density (ind/m2)and biomass (g/m2) of benthic groups at the stations at the same depth.

Winter

Summer Depth

10 m

15 m

10 m

15 m

Density Nematoda Oligochaeta Chironomidae Chaoboridae Other

3616 355 66 1589 5

1255 50 22 1967 0

1026 1521 528 555 5

289 66 49 2805 0

Biomass Nematoda Oligochaeta Chironomidae Chaoboridae Other

0.018 0.295 0.255 3.295 0.001

0.006 0.045 0.017 4.067 0.000

0.005 0.907 2.962 1.682 0.002

0.001 0.050 0.035 9.607 0.000

41 0

ACKNOWLEDGEMENTS This research was supported by a grant from the Italian Ministry of Universit), and Scientific and Technological Research.


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Received: 4 May 1998

Accepted: 31 August 1998