A trophic state index for tropical/subtropical reservoirs (TSItsr)

Um índice de estado trófico para reservatórios tropicais/subtropicais (TSI_tsr )

ABSTRACT

Trophic state is an important property of the aquatic ecosystems as it reflects the anthropogenic influence on water quality and the ecological functioning of rivers, lakes and reservoirs. Trophic state indexes provide an insight on how nutrient and light availability controls phytoplankton development. We analyzed data on total phosphorus (TP, N = 931), chlorophyll a (Chl a, N = 848) and Secchi disk depth (SDD, N = 204) monitored in 18 tropical/subtropical reservoirs from 1996 to 2009 by the Environmental Protection Agency of São Paulo State (Brazil) in a bimonthly basis. Through linear regression with paired data on “TP °ø Chl a” and “Chl a °ø SDD”, we proposed a new trophic state index for tropical/subtropical reservoirs (TSItsr). Based on the annual geometric mean concentrations of TP and Chl a, we also assessed the risk of occurrence of individual episodes (e.g. Chl a ≥ 30  g/L or TP ≥ 50  g/L) within six categories: ultraoligotrophic (U), oligotrophic (O), mesotrophic (M), eutrophic (E), supereutrophic (S) and hypereutrophic (H). The upper boundaries (as annual geometric means) are ( g/L): 15.9 (U), 23.8 (O), 36.7 (M), 63.7 (E) and 77.6 (S) for TP and 2.0 (U), 3.9 (O), 10.0 (M), 20.2 (E) and 27.1 (S) for Chl a. The lower boundaries for the hypereutrophic state were ( g/L) 77.7 (TP) and 27.2 (Chl a). Comparisons with criteria available in the literature suggested that trophic state limits established for temperate systems are not suitable for tropical/subtropical reservoirs and may overestimate their enrichment condition. Restrictions of the TSItsr are discussed in light of the limiting-nutrient concept, the spatial and temporal water quality variability and the use of Chl a as an indicator of phytoplankton density and biomass. The TSItsr may aid in reservoirs management as a starting point for analyzing data on water quality in the tropics/subtropics since this issue is of paramount importance worldwide.

Keywords: Chlorophyll a Eutrophication, Nutrients, Reservoir management, Tropical/subtropical aquatic systems

RESUMO

O estado trófico é uma propriedade importante dos ecossistemas aquáticos, pois reflete a influência antropogênica na qualidade da água e no funcionamento ecológico de rios, lagos e reservatórios. Os índices de estado trófico fornecem uma visão sobre como a disponibilidade de nutrientes e luz controla o desenvolvimento do fitoplâncton. Analisamos dados sobre fósforo total (TP, N = 931), clorofila a (Chl a , N = 848) e profundidade do disco de Secchi (SDD, N = 204) monitorados em 18 reservatórios tropicais/subtropicais de 1996 a 2009 pela Agência de Proteção Ambiental do Estado de São Paulo (Brasil) em uma base bimestral. Por meio de regressão linear com dados pareados de “TP × Chl a ” e “Chl a × SDD”, propusemos um novo índice de estado trófico para reservatórios tropicais/subtropicais (TSI _tsr ). Com base nas concentrações médias geométricas anuais de TP e Chl a , também avaliamos o risco de ocorrência de episódios individuais (por exemplo, Chl a ≥ 30 μg/L ou TP ≥ 50 μg/L) dentro de seis categorias: ultraoligotrófico (U), oligotrófico (O), mesotrófico (M), eutrófico (E), supereutrófico (S) e hipereutrófico (H). Os limites superiores (como médias geométricas anuais) são (μg/L): 15,9 (U), 23,8 (O), 36,7 (M), 63,7 (E) e 77,6 (S) para TP e 2,0 (U), 3,9 (O), 10,0 (M), 20,2 (E) e 27,1 (S) para Chl a . Os limites inferiores para o estado hipereutrófico foram (μg/L) 77,7 (TP) e 27,2 (Chl a ). Comparações com critérios disponíveis na literatura sugerem que os limites de estado trófico estabelecidos para sistemas temperados não são adequados para reservatórios tropicais/subtropicais e podem superestimar sua condição de enriquecimento. As restrições do TSI _tsr são discutidas à luz do conceito de nutriente limitante, da variabilidade espacial e temporal da qualidade da água e do uso de Chl a como indicador da densidade e biomassa do fitoplâncton. O TSI _tsr pode auxiliar na gestão de reservatórios como um ponto de partida para a análise de dados sobre a qualidade da água nos trópicos/subtrópicos, visto que esta questão é de suma importância em todo o mundo.

Palavras-chave: Chlorophyll a Eutrophication, Nutrients, Reservoir management, Tropical/subtropical aquatic systems

Introduction

Artificial dams and reservoirs have been extensively built in Latin America, modifying natural aquatic systems to suit human activities. Such reservoirs were primarily built for energy generation in most cases, but are now used for other purposes, such as water supply, irrigation, flood control, recreation and fisheries. Anthropogenic eutrophication linked with excessive nutrient inputs from domestic wastewater, agricultural and urban runoff is threatening the different water uses worldwide, particularly in tropical and subtropical water bodies due to the population increase and a still poor sewage treatment infrastructure (Ortiz-Jiménez et al., 2006, Cunha et al., 2011). In Brazil, the phytoplankton seasonal variation, the impact of disturbances factors and their implications for Cyanobacteria dominance and the limiting factors for phytoplankton growth in artificial reservoirs were assessed in recent years (Angelini and Petrere, 2000, Sotero-Santos et al., 2006, Rivera et al., 2007, Chellappa et al., 2009, Cunha and Calijuri, 2011, Sant’Anna et al., 2011, Dantas et al., 2012). Eutrophication has also been affecting the reservoirs ecological balance and increasing their environmental vulnerability in Brazil (Figueirêdo et al., 2007, Rangel et al., 2012).

Trophic state indexes promote data grouping and organization, guiding decision making and aiding in the water resources management (e.g. Galvez-Cloutier and Sanchez, 2007, Yang et al., 2012). These indexes provide an insight on how nutrient, light availability and other factors stimulate algal biomass development (usually measured as chlorophyll a, Chl a) and contribute to the increase of the aquatic systems enrichment condition (Duka and Cullaj, 2009). However, there is no consensus on the mechanisms regulating nutrient–chlorophyll relationships in water bodies located in different climate regions (Huszar et al., 2006). Although one common view is that nitrogen limitation may be more frequent in tropical systems (Lewis, 2002), some authors reported that nitrogen did not explain a significant portion of chlorophyll variation in tropical/subtropical water bodies (e.g. Mazumder and Havens, 1998). Therefore, no established rule associating nitrogen or phosphorus limitation (or even co-limitation) with tropical/subtropical or temperate lakes and reservoirs is available thus far (Sterner, 2008).

A trophic state index for temperate lakes was proposed by Carlson (1977) considering empirical relationships among Chl a, total phosphorus (TP) and Secchi disk depth (SDD). This index has been frequently used by researchers and government institutions to indirectly estimate the algal biomass and indicate the eutrophication degree of lentic systems. However, the relationships and the equations for calculating the index should be adapted when applied to aquatic systems different from those from Carlson’s study, otherwise they can lead to misconceptions when proceeding a trophic status assessment. Such adaptations can be related with adding new parameters and variables (e.g. satellite imagery for assessing SDD and Chl a—Olmanson et al., 2008, Sheela et al., 2011; toxic algae densities for evaluating the potential risk of toxins production—Sulis et al., 2011) or performing a specific calibration of the original Carlson’s model based on regressions with local data (Cheng et al., 2001).

Salas and Martino (1991) proposed a simplified total phosphorus model and a trophic state classification for warm-water tropical lakes. These authors reported upper limits for the oligotrophic, mesotrophic and eutrophic status in relation to the geometric means of TP and Chl a concentrations. The classification presented by Salas and Martino (1991) was considered more appropriate to determine the trophic condition of seven lakes in Southeastern Brazil in comparison to that derived from the index proposed by Carlson (Petrucio et al., 2006), reinforcing the importance of establishing specific criteria for tropical/subtropical lakes and reservoirs.

In this paper, we aimed to propose a trophic state index for tropical/subtropical reservoirs (TSI_tsr) as well as limits for TP and Chl a concentrations for six trophic levels: ultraoligotrophic, oligotrophic, mesotrophic, eutrophic, supereutrophic and hypereutrophic. We also evaluated the probability of occurrence of individual critical episodes (e.g. Chl a higher than 30 μg/L or TP higher than 50 μg/L) for each of these categories. Previous studies have presented trophic state indexes and criteria for Southeastern reservoirs (e.g. Salas and Martino, 1991, Lamparelli, 2004), but the revision and update of such indexes are desirable. In this study, we selected reservoirs in the São Paulo State (Brazil) with different water chemistry, topographical features and morphological attributes. Such aquatic systems were comprehensively monitored from 1996 to 2009 to consider both natural and human-induced variability in the water quality, producing a significant number of available data.

Section snippets

Study area

São Paulo State is located in the Southeast region of Brazil (Fig. 1) and has an area of approximately 248,000 km². The state has 41.2 million inhabitants with an associated demographic density of 168 inhabitants/km² (SEADE, 2011). According to Köppen (1936) climate classification, there are seven climate types in the São Paulo territory, whose differences are mainly related to average annual and monthly temperatures and precipitation. The most common climate types in the territory reflect

Results

The annual Gmean concentrations varied between 6.9 μg/L and 124.5 μg/L for TP (N = 931) and 0.7–40.6 μg/L for Chl a (N = 848) within the studied tropical/subtropical reservoirs (Table 3). The minimum and maximum Gmean of the SDD measurements were 0.5 m and 2.4 m (N = 204). Upper concentrations for TP and Chl a were defined for the ultraoligotrophic (U), oligotrophic (O), mesotrophic (M), eutrophic (E) and supereutrophic (S) categories (Table 4). The respective proposed limits considering annual Gmean

Discussion

Many factors are important for regulating the phytoplankton community in lakes and reservoirs, including micronutrients (Vrede and Tranvik, 2006, North et al., 2007), hydrology (Rangel et al., 2012), interactions with other biological communities (Sinistro et al., 2007, Hilt and Gross, 2008, Fonseca and Bicudo, 2010), operational procedures, management decisions in artificial reservoirs (Cunha et al., 2011, Wang et al., 2011) and climate change (Holland et al., 2012, Paerl and Paul, 2012, Sinha 

Conclusions

Trophic state indexes are an important part of the water quality studies. Based on water variables that are relatively simple to measure and quantify, they are easy to calculate and simple to understand and explain. The main highlights of the proposed TSI_tsr and the associated TP and Chl a limits for the ultraoligotrophic, oligotrophic, mesotrophic, eutrophic, supereutrophic and hypereutrophic categories in tropical/subtropical reservoirs are: (i) trophic state criteria for temperate systems

Acknowledgements

We thank FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for the scholarship to the first author (Process Number 2009/50842-2) and for the financial support to the second author (Process Number 2008/55636-9). We are also grateful to CETESB (Companhia Ambiental do Estado de São Paulo) for the dataset used in this paper. Two anonymous reviewers have provided interesting suggestions for improving this manuscript.

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