Submerged Macrophytes, Phytoplankton and Zooplankton in Tropical Reservoir Revista Brasileira de Geografia Física

methods knowledge and technical experimentation. New approaches to aadapted management measures to advance research and to improve water quality is a next step in a near future of reservoirs from Northeast Brazil.


Introduction
Aquatic macrophytes develop and colonize a wide range of natural freshwater environments worldwide, and have high biodiversity in Neotropics (Murphy et al., 2019). In lakes, submerged forms enhance ecosystem processes reducing suspended solids, nutrient uptake and cycling from water column (Carpenter and Lodge 1986). Across latitudes the effects of macrophytes on water quality are recognized (Song et al., 2019). The morphological traits of submerged plants play specific positive feedbacks to water clarity in freshwater ecosystems in the combat of eutrophication (Su et al., 2019). The potential algaecide producing allelopathic substances, became submerged forms a promising management tool to reduce cyanobacteria blooms in lakes-reservoirs (Tazart et al., 2019). Submerged macrophytes are the key point to restore eutrophicated shallow warm freshwater environments and the increased bottomup effects have significant effects on water clarity (Liu et al., 2018).
Alternative equilibria in shallow lakes theory predicts clear water state when submerged macrophytes are dominant, which establish bottomup effect on phytoplankton and interacting simultaneously with zooplankton though top-down control (Scheffer et al., 1993). Zooplankton are recognized top-predator in aquatic food webs and are able to control phytoplankton by herbivory (Carpenter et al., 1985). Studies demonstrate that zooplankton as an indicator of trophic conditions in large reservoirs in Brazil and an indicator of phytoplankton control by herbivory (Brito et al., 2011). Evidences of biomanipulation of food webs in natural ponds and lakes demonstrate that macrophytes influenced the zooplankton community increasing species richness, influence predator-prey interactions (Santos et al., 2020). Mesocosm experiments suggest that submerged macrophyte meadows can establish ecological networks, crucial to determining habitat and interactions for large zooplankton herbivores (Puche et al., 2019). Besides these contributions, is still few evidences about the influence of submerged macrophytes enhance zooplanktonic assemblages, their influence on the water quality and phytoplankton assemblages in Brazilian reservoirs (Rocha et al., 2019). In this context, in cooperation between Brazilian and German institutions, INNOVATE, a transdisciplinary research projects, emerged to find solutions and strategies, improving knowledge for a more sustainable watershed management of reservoirs in the São Francisco River Basin (Siegmund-Schultze, 2017).
The expert and satisfactory water governance requires as a priority water security for the humans survivorship and ecosystem functioning (Tundisi and Tundisi, 2016). In semi arid northeast Brazil, reservoirs series are built for multiple purposes, but the high anthropogenic pressure on these systems enhance eutrophication processes, which in turn hamper the various water uses (Gunkel and Sobral, 2013). In such cases, suitable management measures for water quality improvement must be implemented. For that, it is first necessary to study the mechanisms underlying trophic interactions responsible for the water quality maintenance. Such background knowledge on food chain mechanisms of freshwater systems is crucial for the development of appropriate rehabilitation measures of eutrophicated waterbodies.
In this study it was explored the potential role of submerged macrophytes on promoting water quality (transparency, chlorophyll-a, and low nutrients), zooplanktonic assemblages and low phytoplankton in two shallow bays surround reservoir series in the semi-arid areas from Brazil. We hypothesized, that submerged macrophytes play an important role in determining water clarity, is a factor that shapes and influencing zooplankton and phytoplankton assemblages. In this context, we expect to observe in littoral areas the contribution of submerged plants coverage on improving transparency of the water, low concentrations of nutrients, richness and densities to zooplankton, and low phytoplankton.

Study Area
The São Francisco River is the largest and the most important perennial river of the Northeastern of Brazil, has a length of 3160 km stretching the rainy southwest from Minas Gerais to the dry zone of a Seasonal Dry Tropical Forest in the Caatinga Biome (Maria et al., 2017). The São Francisco River basin covers 640 000 km³ and along the water course eight reservoirs have been constructed for hydroelectric power generation (Hydroelectric Company of the São Francisco River [CHESF 2015]). Itaparica reservoir was constructed in 1988 and is positioned in the middle course of the São Francisco River, between Bahia and Pernambuco states, 290 km upstream from the Atlantic Ocean (Hydroelectric Company of the São Francisco River [CHESF 2015]).
Rainy season (less than 400 mm per year) at Itaparica Reservoir region is generally from January to April with partly heavy rain events (Andrade et al., 2017). This reservoir brings many opportunities for the regional economies, and the water serves multiple sources like power generation, human water supply, recreation, agriculture, aquaculture and artisanal fisheries. The management of water quality in urbanized areas are in need of serious considerations, especially because many external load sources from the land such agriculture, soil erosion, aquaculture, sewage, waste and rainfall . In the last 20 years, the high pressure on water and land use in those area has been contributing to quality impoverishment due to intensive agriculture and aquaculture fomenting eutrophication processes (Günkel et al., 2013).
The main water abstraction of this reservoir is for irrigation purposes . Besides that, artisanal fishery and net cage aquaculture also constitute very important economic activities in the region, although aquaculture leads to severe eutrophication of the water body (Silva et al., 2018). The heterogeneous and dendritic morphometry of Itaparica reservoir shape extensive shallow areas promoting favorable conditions to development of submerged macrophytes that are normally less affected by water currents . Petrolândia (8°59'22.89"S / 38°13'15.91"W) and Icó-Mandantes (8°47'56.54"S / 38°23'17.68" W) bays are the study areas surround Itaparica dam. These environments are shallow, are very vulnerable to eutrophication processes, because the proximity to urban and rural areas which also serve extensive agriculture irrigation and fishery purposes.

Sampling
In this study, were collected samples of water for physical-chemical analyses, of submerged macrophytes (Egeria densa Planch. and Chara guairensis R. M. T. Bicudo), of phytoplankton and of zooplankton in the littoral and the limnetic zones (bay channel) in Petrolândia and Icó-Mandantes bays. Sampling took place in September 2014, January, March and August 2015 in the morning (0900-1200). In each field campaign, the samplings were collected randomly in littoral and limnetic areas of both bays, in depths ranging from 1.5 to 8 m.

Water Quality
Water physical-chemical analyses were conducted at 12 sampling points during the study period. The following abiotic parameters were measured using portable digital equipment (Oakton): water temperature (°C), conductivity (μS/cm -1 ) and pH. The transparency of water was measured with a Secchi disk. Dissolved oxygen (mg/L -1 ) was determined using the Winkler method (Standard Methods for The Examination of Water and Wastewater [APHA 2005]). Nitrogen concentration (TN mg/L -1 ) was analyzed by Kjeldahl, phosphorous concentration by persulphate digestion method (TP mg/L -1 ) (Standard Methods for The Examination of Water and Wastewater [APHA 2005[APHA , 2005b[APHA , 2005c). The Trophic State Index-TSI was calculated according to Carlson (1977) modified by Companhia Ambiental do Estado de São Paulo (CETESB 2004) to classify the water quality of each bay.

Submerged macrophytes
Submerged macrophytes coverage (%) was estimated visually at 12 sampling points distant 500 m each in each bay, which were established parallel to the littoral and in the bay main channel. A vertical rake method (about 5 m²) was used according to Johnson and Newman (2011). The coverage (%) was analyzed through the presence-absence (mi) of species (xi) in each point (MT all points sampled). The coverage was calculated by the following equation xi= (mi/MT)× 100 (Matteucci and Colma, 1982).

Phytoplankton
Samples (250 mL bottle) were collected at 12 sampling points manually in each bay at the subsurface (ca. 0.5 m). The samples for species identification were fixed and preserved with Lugol's solution (Brandão et al., 2011). Abundance evaluation was carried out using inverted microscopy (sensu Utermöhl) at a magnification of 400× for quantification (Standard Methods for The Examination of Water and Wastewater [APHA 2005a]). Density (cell. /mL -1 ) was calculated counting at least 100 organisms per sample. Specific literature was used for phytoplankton taxonomical analyses (Anagnostidis and Komárek 1988;Buchheim et al., 2001;Cavalier-Smith 2004;Komárek and Anagnostidis 1998;Medlin and Kaczmarska, 2004). To quantify phytoplankton biomass as Chlorophyll-a (CHL-a µg.L -1 ), 12 water samples (1 L) were filtered with glass-fibre membranes (Whatman GF/F), acetone was conducted and quantification performed with a fluorometer (Companhia Ambiental do Estado de São Paulo [CETESB 2014]).

Zooplankton
Zooplanktonic assemblages were determined based on an integrated sample, through vertical and horizontal hauls with a plankton standard net (68 μm), totaling 48 samples (24 in each bay). The samples were fixed immediately in formaldehyde solution (4 %) buffered with Sodium Tetraborate (Brandão et al., 2011). The filtered volume was calculated by the following equation (Pinto-Coelho, 2004): VF=πr²d; where VF is the volume filtered by the net, r is the radius of the net aperture (0.15 m), and d is the distance traveled by the net, from depth to the surface of the water column. Organisms were identified to the lowest taxonomic level using specialized literature (Elmoor-Loureiro 1997;Koste 1978aKoste , 1978bPerbiche-Neves et al., 2015;Reid 1985;Segers 2002). Zooplankton richness and density (org. /m -3 ) was estimated by counting the organisms in three 1-mL replicates in a Sedgwick-Rafter-type chamber, using an optical microscope at 400 × magnification (Companhia Ambiental do Estado de São Paulo [CETESB 2000]). The samples with a low number of organisms (N < 200) were fully analysed.

Data Analysis
In order to investigate how submerged macrophytes enhance ecosystem processes in shallow environments, we compared areas with and without plants (littoral and limnetic, respectively) in both bays. A factorial analysis of variance (ANOVA Two-way and One-way Kruskall-Wallis) was conducted to compare transparency of water (Secchi disk), Chlorophyll-a (CHL-a), richness and density of zooplankton (org. / m -3 ) in the study areas. The assumption of normality and homocedasticity was previously verified, and were tested with Shapiro-Wilk and Tukey tests respectively, a posteriori (p <0.05) (Zar, 2010). The analyses were developed with R software version 3.5.1 (2019).

Water Quality
In the bays of Icó-Mandantes and Petrolândia the mean water temperature, dissolved oxygen and electrical conductivity in the sampling periods were similar in both environments. Mean Nitrogen (TN mg/L -1 ) and Phosphorus (TP mg/L -1 ) were present in low concentrations in both bays (Table 1). Water transparency (Secchi disk depth) was generally higher in Icó-Mandantes bay (mean of 4m) than in Petrolândia (mean of 3m). Besides that, the difference between both areas was not statistically significant (H = 1.15, df= 1, p= 0.28) ( Figure 1). Moreover, the TSI of both bays was 65 and 63 for Icó-Mandantes and Petrolândia respectively, which corresponds to Supereutrophic and Eutrophic water quality conditions (Table 1).

(Eutrophic)
Macrophytes Egeria densa Planch., was the dominant aquatic plant identified covering the large littoral area of Icó-Mandantes (shallower than 7 m water depth). In this bay, Chara guairensis R. M. T. Bicudo was identified covering only shallow sandy areas (< 1.0 m water depth). The highest percentage of coverage of E. densa (< 3 m depth) in Icó-Mandates was observed in the littoral areas (66%), different from the limnetic zone (16%). In Petrolândia E. densa was had low coverage areas in the littoral (40%) and limnetic zone (8%). E. densa offers shelter for aquatic invertebrates like shrimps and mollusks (Biomphalaria glabatra, Say, 1818) that were observed to be attached to stems and leaves of this plant in both bays (pers. obs.). With this background, it was then established that the limnetic zone are areas of these bays with at least 6m water depth, and littoral zone are the regions shallower than 6 m water depth, where Egeria densa or Chara guairensis were constantly present.

Discussion
Our study brings new ecologic and taxonomic data from São Francisco River in Brazil. Few studies have focused reservoirs in the semiarid area of Brazil to address the role of submerged plants on the water quality, and the influence of zooplanktonic and phytoplankton (Rocha et al., 2019). Itaparica reservoir is under a very high anthropogenic pressure and that was observed on both bays, which revealed a high trophic index, correspondent to eutrophic waterbodies. Such index values were corroborated by the massive development of the submerged macrophyte Egeria densa in both bays, which is known for its pioneer and rapid growth behavior. The eutrophic state was also disclosed by the prevalence of mostly Cyanobacteria species in the phytoplankton community of these bays. Despite the low macrophytes richness in the study areas, the potential of submerged forms on improving zooplankton assemblages was nonetheless confirmed in this study. The species richness and densities of zooplankton observed in littoral regions shows the importance of submerged macrophytes beds in tropical freshwater systems. This information is of crucial value for future rehabilitation measures of these systems.
A dense coverage of submerged macrophytes congregate large microcrustaceans such Daphnia and calanoids in the littoral of the study areas. Cladocerans like Bosminidae and Daphnids were identified, and constitute an important group of algae consumers, which can feed on and reduce phytoplankton growth. Diaptomidae and Cyclopidae are omnivorous and are known to feed on microphytoplankton (Brito et al., 2011). Corroborating this evidences, a multi-interaction network models proposed suggest macrophytedominated lakes are the key element to provide habitat to large herbivores (such as Cladocera or Copepoda) (Puche et al., 2019). Macrophytes induce positive changes in water quality, and influenced the zooplankton by contributing to increased species richness, especially to small-and medium-sized littoral cladocerans (Santos et al., 2020). In spite of the poor richness of copepods, the high densities of Calanoida, Cyclopoida and nauplii in Petrolândia (2 203-org. /m -³ ) probably is a good bioindicator by maintaining low algae biomass.
Low densities of Chlorophyta were observed, corroborate the bottom-up effect of Egeria densa and the grazing pressure on the edible algae by the smallest rotiferans filter feeders such Brachionus, Keratella and Synchaeta. Submerged form competing with light, nutrients and allelopathy with phytoplankton in natural and manipulated lakes (Scheffer et al., 1993;Liu et al., 2018;Tazart et al., 2019). Besides that, inedible Cyanobacteria showed great density on phytoplankton assemblages in both bays. On the one hand, this can be explained by eutrophication processof these bays (Günkel et al., 2013). On the other hand, a higher abundance of inedible cyanobacteria or species which can form large colonies (ex. Fragilaria crotonensis) also indicates that a high grazing pressure from zooplankton on smaller species is probably taking place in these waterbodies.
Itaparica was constructed to generate power hydroelectricity, but the water body serves multiple purposes to supply urban areas, such as food production and economy development. Studies on Itaparica reservoir have evidenced the necessity to manage littoral areas, which discharges and nutrients from the lands brings high eutrophication risks (Günkel et al., 2013;. The high coverage of submerged macrophytes is a key point for the management of these bays. The removal of massive monospecied macrophytes beds is a practical current tool to manage eutrophic freshwater environments . Maintenance or restoration to guaranty water quality assessment in reservoirs with proper allocation to natural needs for the human uses and their economies is a next step and key initiative for the future water governance (Tundisi and Tundisi, 2016).
New aquatic ecology techniques to combat eutrophication process, though manipulate food webs and submerged macrophytes, are gap to scientific and Technical experiments in Semi-arid areas from Brazil (Rocha et al., 2019). Icó-Mandantes and Petrolândia are good potential systems for restoration and the background knowledge of the trophic interactions techniques is urgent, and a promise tool to improve new approaches to guaranty water quality supply for million people. In the Northeast region with extreme water scarcity needed, knowledge for well-founded and adapted management measures for the numerous reservoirs the São Francisco River Basin.