Sewage input effects on the macroinvertebrate community associated to Typha domingensis Pers in a coastal lagoon in southeastern Brazil

This study was carried out at Imboassica Lagoon, located in an urban zone in the municipality of Macaé, Rio de Janeiro state, Brazil. This lagoon has been subject to anthropogenic impacts due to the increasing city population, such as the input of sewage. Areas of variable degree of anthropogenic influence in the lagoon were compared regarding the structure of the macroinvertebrate community associated to Typha domingensis leaves. For sampling, we used 35 x 20 cm net plastic bags, with 6.8 mm mesh containing T. domingensis leaves for colonization. Two different sampling stations were selected: station A, under direct input of sewage; and station B with lesser sewage influence. The bags were removed after 20, 40 and 75 days of colonization. For each sample the Shannon-Wiever Diversity, Pielou Evenness, Jaccard Similarity Indices, Correspondence Analysis and taxonomic richness were calculated. A total of 31,874 individuals were sampled, belonging to 34 taxa. The main taxonomical groups were: Oligochaeta (41%), Chironomidae (40%), Ancylidae (4.6%), Polymitarcyidae (4%) and Thiaridae (3%). At station A, the taxonomic richness, the Evenness and Diversity values were lower than in station B. On the other hand, the total density was three times higher in station A than in B. It was already possible to discriminate the community structure of each sampling station in the first sampling. Trichoptera and Ephemeroptera were the main exclusive groups of station B and are considered good water quality indicators due to their high sensibility to contamination. The major contribution to discriminate between the macroinvertebrate communities of the two sample stations came from Chironomidae, Oligochaeta and Ephemeroptera.


Introduction
Coastal lagoons are abundant on the Brazilian coast, especially in Rio de Janeiro and Rio Grande do Sul States (Esteves, 1998a).Such lagoons show primary productivity comparable to estuarine environments and have many important resources for human exploitation (e.g., fishing, production of macrophytes for feeding, fertilizers, handcrafts and tourism) (Esteves et al., 1984).Contrasting to temperate lakes, in tropical coastal lagoons the primary productivity of macrophytes and associated periphyton is relatively more important than the phytoplankton productivity.Aquatic and semi-aquatic macrophytes usually contain larger macroinvertebrate densities than other substrates (Minshall, 1984).Macrophytes are substratum for the main food sources exploited by macroinvertebrates: periphyton and particulate organic matter (Ward, 1992).Moreover, the macrophytes produce debris and increase the habitat heterogeneity leading to higher diversity of the macroinvertebrate fauna (Hynes, 1970;Minshall, 1984).
The coastal lagoons of northern Rio de Janeiro State, Brazil have undergone various anthropogenic impacts since the first half of the XX century (Soffiati, 1998).The sewage input may act as an energy source or a stress factor for an ecosystem, altering its productivity and community development.An effect associated to the stress hypothesis is the decrease of species diversity, as a result of the taxonomic richness decrease and the dominance increase of a few more resistant species (Odum, 1985(Odum, , 1988)).Freshwater macroinvertebrates have been frequently used in water quality studies.The main advantages regarding their use in these studies are the great number of species that may be sensitive to environmental stress, their wide distribution in various freshwater habitats and the relatively sedentary behavior and short life cycle in relation to fish, which facilitates the detection of temporal changes (Rosenberg and Resh, 1993).
In this study, a natural substrate, leaves from Typha domingensis Pers (Thyphaceae), was used for benthic macroinvertebrates colonization.T. domingensis is the dominant macrophyte surrounding the Imboassica Lagoon and represents the main vegetable substrate to periphyton and invertebrates.The sewage input causes eutrophication and increases suspension detritus that deposit on the macrophyte leaves.The Imboassica lagoon is a mosaic differing in the intensity of domestic sewage input that resumes differences on the limnological and biotic parameters (e.g., suspended detritus, ammonia concentration, fecal coliforms concentrations) (Henriques de Oliveira, 2002;Petrucio and Furtado, 1998).Responses of organisms according to their habits and characteristics deal with differences in community parameters such as dominance, richness, diversity and evenness (Resh and Rosenberg, 1984).The aim of this study is to assess if the variation in raw domestic sewage input influences the macroinvertebrate community structure associated to T. domingensis leaves.

Study area
The Imboassica lagoon (22° 20' and 22° 25' S; 41° 45' and 41° 55' W) is located in the urban zone of the municipality of Macaé, Rio de Janeiro state, Brazil.It presents a total area of 3.26 km 2 , maximum width of 1.3 km, length of 5.3 km, average depth of 1.09 m and average volume of 3.53 km 3 (Panosso et al., 1998).Due to the rapid growth of Macaé since the 1960's, the Imboassica Lagoon has been subjected to discharge of raw domestic sewage in different points and about 20% of its area was landfilled for real estate use (Esteves, 1998b).As a result of these anthropogenic factors, there is a continuous reduction in the lagoon depth leading to an increase in its water surface in relation to volume (Esteves et al., 1984).The reduction in the area/volume rate helps to establish macrophytes.Typha domingensis, the Cyperaceae Eleocharis cf.fistulosa (Mart.)Solms and various species of Poaceae occupy roughly 38% of the total area of the lagoon (Furtado, 1994).
Two sampling stations subjected to different intensity of raw domestic sewage input were chosen, both in the coastal zone, close to the macrophytes stand (Figure 1).Station A, located close to the main sewage inflow, presented fecal coliform concentrations higher than 2,500 Most Probable Number (MPN)/100 mL, an average oxygen concentration of 8.1 mg.L -1 , an ammonia concentra- tion between 11.90 and 26.19 mg.L -1 , a pH slightly acid (6.8) and a salinity around 1 S. Station B, representing the control station, is located near the Imboassica River at about 3.5 km from the station A (Figure 1).It presented fecal coliform concentrations around 500 MPN/100 mL, an average oxygen concentration of 5.0 mg.L -1 , an ammonia concentration between 0.31 and 2.38 mg.L -1 , a pH between 6.7 and 7.3 and salinity values not surpassing 0.9 S.

Sampling design
The benthic macroinvertebrate samples were obtained from colonizing green leaves of Southern Cat-tail, T. domingensis.Emerged parts of the T. domingensis leaves were cut and placed in net plastic bags, with a size of 35 x 20 cm and mesh 6.8 mm.Each bag contained 100 g of leaves.In each station, nine bags were installed, distributed in three sampling sets, with a total of 18 bags in the two stations.The sampling sets were removed after three different colonization periods (20, 40 and 75 days) between November 2000 and January 2001.These time intervals are based on other macroinvertebrate colonization studies and are related to minimum times necessary for establishment and stabilization of the community (Nessimian and De Lima, 1997;Walker, 1998;Gonçalves-Jr., 1999;Kuhlmann, 2000).The leaves were washed and the material retained in sieves of 187 µm mesh was sorted and identified under a stereomicroscope with 160x magnification.All specimens were deposited in the Coleção do Departamento de Zoologia da Universidade Federal do Rio de Janeiro.

Data analysis
The shannon index (H'), Evenness (J) (Magurran, 1988) and richness (number of taxa) were measured for each sample (up to the smaller possible taxonomic level).Hutcheson's method, described by Magurran (1988), calculates the t-value and degrees of freedom, testing the significance of differences in diversity between the sampling stations and colonization periods.
The Jaccard Similarity Index was used to evaluate the qualitative macroinvertebrate community similarity among samples.Correspondence Analysis (Ludwig and Reynolds, 1988) was performed on a data matrix of macroinvertebrate densities, with the help of the NTSYS program, version 1.70 (Rohlf, 1992).
In the Correspondence Analysis, the first two axes explained together 80% of the total variation.Axis I (60% of the variation) might be interpreted as the varia- tion in water quality (Figure 3).The samples of station A were positively correlated to Axis I, whereas samples of station B were negatively correlated (Figure 3).The structure of the fauna varied more between stations than among periods of colonization.In station A, the abundance of Oligochaeta was about 20 times higher than in station B, contributing with 21% to the formation of Axis I.An exclusive taxon for station B, Campsurus melanocephalus Pereira and Da-Silva, 1991 (Ephemeroptera; Polymitarcyidae), showed a negative correlation with Axis I and contributed with 31% to its formation.Other organisms negatively correlated to Axis I were Coenagrionidae (3%), Cladocera (6%), Copepoda (5%), Hirudinea (7%), Ostracoda (4%) and Cyrnellus (5%) (Trichoptera: Polycentropodidae).
Axis II (20% of the variation) might be interpreted as the quality of the substrate available for colonization.Chironomidae (50%) and Oligochaeta (31%) presented the highest contribution for this axis, both representing some of the most abundant macroinvertebrate organisms found in the two sampling stations.They also presented the highest variation among different colonization periods.Chironomidae was more abundant in the sample set removed after 75 days of colonization at station B, in comparison to the shorter periods (20 and 40 days).Otherwise, the highest abundance at station A was found in the sample set removed after 40 days and the lowest abundance at the 75 day sample set.Oligochaeta also presented decreased abundance in the 75 day sample set at both studied stations.Other organisms showing decreasing abundance in the longest colonization period (75 days) were Hirudinea, Hydrobiidae (Gastropoda), Copepoda and Cyrnellus, contributing with 3, 2, 3 and 4%, respectively, to Axis II.Such results were more significant for station A, where the individual abundance for several taxa showed a higher decrease over time.The observed reduction in abundance throughout the colonization period in station A may be a result of the advanced degradation of the leaves (they seemed softer and thinner when observed visually and when touched) in that station, as compared to those at station B.

Discussion
The rapid establishment of the macroinvertebrates community in station A demonstrates the role of sewage as an energy source for some groups.Oligochaeta and Chironomidae were the main organisms found in this samping station.Chironomidae is one of the most abundant groups of the benthic fauna in freshwater en- vironments (Coffman and Ferrington, 1996) and may be found in several different habitats (Pinder, 1986).The occurrence of Chironomidae as a dominant group has been observed in many studies on macroinvertebrate communities of natural and degraded environments (e.g., Nessimian and De Lima, 1997;Correia, 1999;Gonçalves-Jr, 1999;Araújo, 2000;Silveira, 2001).
Chironomus and Goeldichironomus, the dominant genera of Chironomidae in station A, are considered as organisms with a high tolerance to habitats presenting high contents of organic matter and low oxygen levels (Ruse and Wilson, 1995).The abundance of these genera can be related to their collector-gatherer and, possibly, scraper feeding habits, as both kinds of organisms use debris as the main food item (Henriques-Oliveira et al., 2003).The increased abundance of Oligochaeta is correlated with environments subjected to a high input of domestic sewage, as pointed out by Navas-Pereira and Henrique (1996).Thus, organisms with a positive feedback response to sewage input, such as Oligochaeta, Chironomus and Goeldichironomus represent most of the disparity in the abundance of benthic organisms between the stations.Despite the highest abundance of organisms in station A, the greatest richness was found in station B. The absence of some taxa in station A could be related to their low tolerance to organic pollution.According to Grosse et al. (1986), some Amphipoda families can be sensitive to high amounts of suspended organic material.Ephemeroptera and Trichoptera are often related to high or intermediate environmental integrity, as they are used as bioindicators in biological monitoring studies in freshwater environments.These taxa are, for example, very dependent on oxygen availability (Rosenberg and Resh, 1993;Lemly, 1982;Barbour et al., 1996;Silveira, 2001).The presence of Trichoptera and Ephemeroptera exclusively in station B indicates that these taxa present a high enough sensibility to react to the different amount of degradation found in the sampled stations.
Enrich-Prast and Fernandes (1998) found smaller rates of biological fixation of nitrogen by the periphyton associated to T. domingensis in station A of the Imboassica Lagoon.Such results were related to the formation of toxic compounds and to the inhibition of nitrogen fixation due to the presence of high nitrite and ammonia concentrations in polluted environments.The increase in the concentration of nitrogen and proteins from activities of bacteria, fungi, and epiphytic algae, may increase the attractiveness of the periphyton to macroinvertebrates (Suren and Lake, 1989).Thus, low values of nitrogen fixation could reduce the attractiveness of the periphyton, decreasing the diversity of invertebrates that feed upon these organisms.Fernandes (1998) observed smaller taxonomic richness of the periphytic community associated to leaves of T. domingensis in station A. The periphytic cover is considered an important factor influencing the structure of the macroinvertebrate community (Van Den Berg et al., 1997;Albertoni et al., 2001;Nessimian and De Lima, 1997;Correia, 1999).Caenis cuniana Froehlich, 1969 (Ephemeroptera: Caenidae) feeds mainly on algae, vegetable fibers, and organic debris (Francischetti et al., 2001).Amphipoda species associated to the aquatic vegetation can feed on epiphytic algae, small animals and organic debris (Pennak, 1978;Grosse et al., 1986).Most Gastropoda are herbivores, including vegetable tissue (live or dead) and periphytic algae in their diet (Pennak, 1978), as exemplified by Physidae and Hidrobiidae (Suren and Lake, 1989;Cardoso et al., 1993).The pollution influence on the distribution of the benthic macroinvertebrate species can be attributed to change in food availability or to toxins associated to food (Thorne and Willians, 1997).The greatest herbivore richness found at station B can corroborate the results pointed out by Enrich-Prast and Fernandes (1998) and Fernandes (1998) on periphyton richness in stations A and B.
The higher availability of organic matter close to the sewage discharge points may lead to the increase of abundance and biomass in the macroinvertebrates, as also observed in the same lagoon by Gonçalves Jr et al. (1998) and Albertoni et al. (2001).However, in the present study, the observed diversity values were smaller, in response to the richness decrease and the largest dominance of the more tolerant groups in station A. In studies of environmental biological integrity, higher values of evenness and richness may represent better environmental integrity.The increase in abundance and biomass of a few taxa may represent a negative effect of pollution, as only the more tolerant organisms are able to use the input of organic material as an energy subsidy.

Table 1 .
Total abundance of benthic macroinvertebrates colonizing Typha domingensis leaves during different colonization periods in stations A and B of the Imboassica Lagoon, Macaé, RJ, November 2000 through January 2001.
Figure 3. First two components from Correspondence Analysis among colonization periods of Typha domingensis leaves by benthic macroinvertebrates in sample stations of the Imboassica Lagoon, Macaé, RJ, November, 2000 through January, 2001.Sample stations: A and B; colonization periods: 20, 40, and 75 days.

Table 2 .
Richness (S), Shannon index (H') and evenness (J) of the benthic macroinvertebates colonizing Typha domingensis leaves during different periods in stations A and B of the Imboassica Lagoon, Macaé, RJ, November 2000 through January 2001.