Shallow Tropical Lake Research Articles

Analysis on the changes of zooplankton in the restoration of urban lake water

The combination of aquatic vegetation restoration and biological manipulation had become one of the important ways of ecological restoration of lake water. Zooplankton was closely related to aquatic plants, which affected the species composition, density and diversity index of zooplankton. Zooplankton was also a part of biological manipulation, which helped to control phytoplankton and improved the transparency of water body. Through the analysis of the role of zooplankton in ecological restoration and the response of zooplankton to the ecological restoration of water body fully reflected the complementary relationship between biological manipulation and aquatic vegetation restoration. Combined with the practice of zooplankton changes in the ecological restoration of typical tropical shallow city lake, it was expected to provide basis and support for the restoration of urban lake.

The relative importance of regional and local factors in shaping zooplankton diversity in high-altitude tropical shallow lakes

In Brazil, although, high-altitude shallow lakes comprise large parts of conservation units and are considered highly important environments for biodiversity studies, little is known about the factors that regulate the zooplankton community. In the present study, we sought to identify the effect of regional (hydroperiod and altitude) and local (lake area and macrophyte cover) factors on the zooplankton communities in eight shallow high-altitude lakes (from 1080 m a.s.l. to lakes above 1200 m a.s.l.) in the Espinhaço Mountain Range (Brazil). Of the 116 zooplankton species identified, 66 (56%) showed a high degree of spatial aggregation, occurring in only a few lakes. Accessory species occurred predominantly in small lakes, while rare species were better represented in the larger lakes, suggesting species sorting associated with the niche effect. Zooplankton richness was strongly influenced by hydroperiod, indicating that temporary lakes can congregate species resistant to the environmental filter of the drought, playing an important role in the diversification of the zooplankton community. We identified environmental heterogeneity (macrophyte cover) as the factor that most influenced the species diversity: zooplankton responded positively to increased macrophyte cover, confirming that the presence of aquatic plants constitutes a strong environmental filter to explain the variation of the zooplankton community. Zooplankton species composition and abundance differed among lakes, indicating a certain degree of heterogeneity in assemblage structure, but no influence on zooplankton diversity was observed. Our results provide evidences that altitude did not influence zooplankton diversity and this may affect the regional diversity (ß diversity) of tropical high-altitude lakes, and merits further study.

The overriding role of hydrological factors on zooplankton community: evidence from a shallow tropical reservoir (Koka, Ethiopia).

Due to changes in global climate and local hydrological variability, land-water interactions are considered the prevailing processes underpinning aquatic ecology. Periodic meteorological conditions could have significant effect upon freshwater plankton communities by modifying the relative influences of biotic and abiotic structuring factors. However, the influence of unusual meteorological events, such as the El-Niño Southern Oscillation cycles, and advection on zooplankton communities of tropical shallow lakes and reservoirs are not fully understood. The present study, therefore, aimed to explore the relationship between hydrological factors and zooplankton community structure in Koka Reservoir, Ethiopia. Field measurements and collection of water samples and zooplankton were performed at monthly intervals from January to August 2016 at three sampling sites along a stretch of the reservoir from the river mouth to the center and the outlet (dam). The results showed post-El-Niño hydrological events (i.e., elevated riverine runoff and higher levels of turbidity) and associated advective losses can disrupt zooplankton community structure at the river mouth site, particularly during the rainy period. The results suggest that integrated watershed management activities are important aquatic resource management options in small- and medium-sized lakes and reservoirs.

Study on the relationship of ecological effect in the ecological restoration of urban lakes

the combination of biological manipulation and aquatic vegetation restoration had become an important way of ecological restoration of lake water. The research on the relationship among fish, zooplankton and aquatic vegetation was helpful to lake ecological restoration. By investigating the distribution of fish, aquatic vegetation and zooplankton in some urban lakes in the South of the five ridges, the abundance and relative abundance of Cladocera and Copepoda were generally lower in the lakes where fish was farmed, while the abundance of Cladocera and crustacea was relatively higher in lakes with hydrophytes. The ecological restoration of typical tropical shallow urban lake showed that the combination of aquatic vegetation restoration and fish regulation was an effective measure for the control of tropical shallow eutrophication urban lake. It provided a reference for how to take biological control measures to restore the water body and how to maintain the stability of the ecosystem after the ecological restoration.

Day time variations of zooplankton species composition: observations from the wetlands of Assam, India

Abstract: Aim An attempt to justify the variations in the species composition of zooplankton over the day time period was made using the freshwater lakes of Assam, India, as a model study area. Methods The water bodies were sampled using a conical plankton net five times between 6 am and 6 pm in a day during July 2014. Apart from diversity analysis (SHE analysis), discriminant function analysis and a regression tree were built to justify the variations in the zooplankton species composition in the different time periods in a day. Results Repeated sampling during day time indicated the presence of 34 zooplankton species with varied composition over the different time periods. The species richness and abundance of Copepoda, Cladocera and Rotifer differed significantly revealed through the ANOVA (for rotifers – F4, 170 = 28.792; for cladocerans – F4, 170 = 16.343; for copepods - F4, 170 = 9.027). Apparently, the rotifer and copepod abundance was prominent during the early (6am) and late time (6pm) periods of a day while the cladoceran abundance was significant during the middle time periods (12 noon to 3pm) of a day. Variations in the relative abundance of the zooplankton species within a day were sufficient to discriminate the time periods significantly. The regression tree indicated the rotifer abundance (node 1), followed by the cladoceran and the copepod abundances (node 2). The nodes representing cladocerans explained 30.86% of the data while the copepod explained 69.14% data as a split from the rotifer groups. Conclusions A correspondence of high and low abundances of rotifers and cladocerans was characteristic of the species composition of the zooplankton in the study area. The variations in the species composition suggests that the zooplankton exhibits movements either in horizontal or vertical directions induced by physical and/or biological factors. Further studies may be adopted to elucidate the effects of the predators and environmental variables that shape the community of the zooplankton in shallow tropical lakes.

Zooplankton species distribution, richness and composition across tropical shallow lakes: A large scale assessment by biome, lake origin, and lake habitat

Assessing zooplankton biodiversity is essential to support freshwater management/conservation programs. Here, we investigated the zooplankton community structure from 180 shallow lakes in northeastern Brazil and analyzed them according to biome (Atlantic Forest or Caatinga), the origin of ecosystems (natural or man-made lakes), and habitat type (pelagic or littoral). Additionally, we provided an updated list of zooplankton species. We registered 227 species (137 Rotifera, 65 Cladocera, 25 Copepoda). The most common species of each major group among all lakes were the cladoceranCeriodaphina cornuta, the rotifersBrachionus havanaensisandLecane bulla, and the copepodTermocyclops decipiens. Species related to aquatic vegetation, as the Lecanidae rotifers and phytophilous cladocerans, were more frequent along Atlantic Forest biome and natural lakes. On the other hand, species that are bioindicators of eutrophic waters were more common at the Caatinga biome and man-made lakes. Atlantic Forest and Caatinga biomes had similar species richness, but different community compositions for all zooplankton groups, reinforcing the Caatinga significance for the Brazilian aquatic biodiversity. The type of habitat was the most important factor structuring species richness, with higher richness in the littoral region when compared to the pelagic. A result of many unique species of Cladocera and Rotifera associated with the aquatic vegetation were observed. The findings demonstrated that conservation/management plans cannot generalize zooplankton species distribution across different biomes, origins and even within a single lake, between the pelagic and littoral zones.

Eutrophication Bifurcation Analysis for Tasik Harapan Restoration

Regime shift is characterized by large, abrupt and persistent changes in ecosystem structure and dynamics. Bifurcation analysis is commonly used to identify regime shift equilibrium states and to distinguish their stability characteristics. Eutrophication in lake, a regime shift from clear-water oligotrophic state, is a stable equilibrium state that can persist for long duration. Characterized by undesirable turbid water condition, eutrophication has been known to impair valuable ecosystem services provided by lakes worldwide. The high incidence of eutrophication in Malaysian lakes (62%) mandates urgent need for lake restoration. The three-fold objectives of this paper are (1) to develop a mathematical model for analysing bifurcation criteria in regime shift, (2) to identify regime shift thresholds and (3) to propose effective ecosystem management strategy for shallow tropical lakes such as Tasik Harapan. A mathematical model consisting of four compartments: algae, phosphorus, dissolved oxygen and biochemical oxygen demand is formulated to analyse the eutrophication dynamics in the highly eutrophic Tasik Harapan (TH), a small shallow lake in Universiti Sains Malaysia (USM). Bifurcation analysis is performed by means of XPPAUT to identify the regime shift thresholds and to determine the type of lake response. Identified as irreversible, the eutrophication state of TH mandates an urgent lake restoration program to remove nutrients in the lake. Two restoration methods for reducing nutrients are assessed: (1) flushing of lake water and (2) hypothetical application of the invasive water hyacinth. Bifurcation analysis indicates that a flushing rate exceeding 0.042 day-1 is needed to restore TH to oligotrophic state. A complicated strategy of water hyacinth application would reduce the algae concentration from 300 μg/l to 120 μg/l after 9.6 years. A severe shortfall is the complicated and uncertain process of frequent removal of water hyacinth to prevent the lake from being overwhelmed by the invasive plants. The feasibility and sustainability of these two lake restoration methods are discussed. The insights gained would be useful to the relevant authorities in determining and implementing the best remediation measures for TH.

Towards integrated management of a shallow tropical lake: assessment of water quality, sediment geochemistry, and phytoplankton diversity in Lake Palakpakin, Philippines

The limited carrying capacities of shallow tropical lakes render them more vulnerable to ecological problems like eutrophication. Unregulated human activities such as unsustainable aquaculture and urbanization can alter ecosystem dynamics rapidly, and this warrants more comprehensive researches than what has been previously conducted. Here, we presented an integrated assessment of the nutrient dynamics, phytoplankton diversity, and sediment geochemistry in Lake Palakpakin, a shallow tropical lake of volcanic origin, to understand its deteriorating ecological state. Water, phytoplankton, and sediment samples were collected, and in situ water quality measurements were done during wet and dry seasons in four critical areas in the lake, namely, the inlet, center, sanctuary, and outlet. Results revealed that high light extinction coefficient (1.13m-1), high turbidity (28NTU), high phosphate concentration (> 2.0.5mg/L), and the abundance of Microcystis aeruginosa, Anabaena helicoidea, and Lyngbya sp. indicate that from a relatively healthy lake in 2008, Lake Palakpakin has become a eutrophic to hypereutrophic freshwater body. High concentrations of available nutrients such as N and P were detected in the center and sanctuary sediments, which drive the internal nutrient loading in the lake. We recommend that management efforts be directed towards a whole-ecosystem approach in addressing the problem of eutrophication, especially in shallow tropical lakes.

Dissolved organic N in shallow tropical lakes and reservoirs: Contribution to total dissolved N and relationships with eutrophication

AbstractDissolved organic nitrogen (DON) plays important biogeochemical and ecological roles in aquatic systems. However, little is known about its importance in small tropical lakes and reservoirs. We hypothesized that (a) DON is higher in these systems than dissolved inorganic N (DIN), and that (b) relative contributions of DON and DIN to the total dissolved nitrogen (TDN) pool decrease and increase, respectively, with trophic status. To test these hypotheses, we measured DON and DIN in six lakes and two reservoirs at the Atlantic forest‐Cerrado savanna transition in Southeast Brazil, in the rainy and the dry season. With one exception in each season, all oligo‐ and mesotrophic systems had DON:DIN ratios >1. DON:DIN ratios were negatively correlated with total phosphorus (TP). This pattern was due to high DIN, mainly ammonium, in eutrophic and hypertrophic systems. DON:DIN ratios <1 occurred at TP higher than 103 and 104 µg L−1 in the rainy and dry season, respectively. We found a general dominance of DON in near‐natural lentic systems at the Atlantic forest‐Cerrado savanna transition, which was more pronounced than that reported for other regions. Human alterations to the TDN pool may affect biological communities and ecosystem functioning of these systems.

Long-Term Habitat Degradation Drives Neotropical Macrophyte Species Loss While Assisting the Spread of Invasive Plant Species

Macrophyte invasive species spreading is one of the most pressing threats to tropical shallow lakes. Yet, studies addressing the full extent of biotic and abiotic changes that may occur in response to invasive species are poorly documented. Less known is how other human-induced stressors such as eutrophication and lake draining may interact over time with invasive macrophytes to jointly influence biodiversity loss. We combined ecological and limnological observations with paleoecological data from a eutrophic Neotropical shallow lake, Fuquene Lake, Colombia, to provide information on the current and long-term (decades-centuries) dynamics of spread of two well-established invasive plants Eichhornia crassipes and Egeria densa. At present-day, areas dominated by Egeria and Eichhornia were unique in macrophyte structure and differed from those areas having a mixture of macrophyte species or open waters. Eichhornia areas encompassed 14 macrophyte species and had turbid (secchi=19 ±6 cm) and poorly oxygenated (3.94 ±2.61 ppm) waters. Egeria areas had clearer (secchi=51 ±12 cm) and better-oxygenated (6.06 ± 2.4 ppm) waters supporting six macrophyte species. Historical macrophyte community shifts were linked to eutrophication and hydrological alteration exemplify by losses of charophytes and bryophytes before 1500 CE and subsequent reductions in Nymphaea sp., Potamogeton illinoensis and Najas guadalupensis in the early 1900s (lake draining). E. crassipes (since 1500 CE) and E. densa (early 1900s) occurred well before suggested dates of introduction (1950s and 1990 respectively). Yet, they severely expanded since the 1990s along with Azolla filiculoides in response to an inflow waters diversion scheme and heavy nutrient load. Results suggest that the spread of Eichhornia and Egeria was not responsible for historical resident species loss. Nevertheless, their current domination is exerting synergetic and antagonistic secondary effects on plant assemblages through habitat modification, competitive exclusion and promotion of habitat spatial heterogeneity across the lake. Equating macrophyte species loss with invasive plants spreading in degraded lakes could be therefore misleading. Aggressive species like Eichhornia, Azolla and Egeria are suggested to require eutrophic environments to spread, thus, management actions should focus on controlling nutrient load. Our study demonstrates the need of addressing long-term approaches to fully-understanding the effects of invasive macrophyte spread.

Epipelon responses to N and P enrichment and the relationships with phytoplankton and zooplankton in a mesotrophic reservoir

The phototrophic epipelon has been suggested to play an important role in ecosystems, especially those with shallow depths; however, only a few studies have investigated this function. Nutrient availability has been shown to be a determining factor for autotrophic interactions and can potentially interfere with the food web, as eutrophication. Thus, we evaluated the responses of epipelon, phytoplankton and zooplankton to combined and isolated N and P addition, during the enrichment period (14 days) and after 12 days with no enrichment. It was hypothesized that P addition (the limiting nutrient) should decrease the photosynthetic potential of the epipelon, due to the rapid increase in phytoplankton and zooplankton biomass, which can strongly attenuate light, and that the opposite effect would be observed after a period with no enrichment. We developed an in situ experiment with combined and isolated N and P enrichment at open-bottom mesocosms. The addition of P, individually and combined, augmented phytoplankton chlorophyll-a concentrations during the enrichment period, while zooplankton density only responded positively after day 14. After 12 days with no enrichment, the phytoplankton chlorophyll-a and zooplankton density decreased. While P enrichment had no significant effect on epipelon chlorophyll-a, there was a significant increase in the photosynthetic potential detected 12 days after the enrichment was stopped. In conclusion, the present study demonstrated that P enrichment reduces the photosynthetic potential of epipelon, and that variations in nutrient availability can modulate relationships among phytoplankton, zooplankton and epipelon. Drastic changes in the growth and development of the phototrophic epipelon, due to the input of nutrients, could directly impact the functioning of shallow tropical lakes and reservoirs.

Macrophytes in tropical shallow lakes: An important food item to benthic entomofauna or an underused resource?

AbstractAlthough macrophytes are known to increase benthic diversity in lakes, the importance of this resource as food for the insects living at the bottom of these ecosystems are still poorly understood. This study assessed the diets of benthic Chironomidae and Campsurus (Ephemeroptera) in two environments: a lake with macrophytes (M+) and another without macrophytes (M−). We expected a differential use of food resources in M+, where plant tissue is particularly important for the aquatic insects’ diet. The diet of 734 individuals from 16 taxa were analyzed. Contrary to expectations, benthic insects consumed low amounts of plant tissue. This finding led us to investigate whether the presence of macrophytes in lakes would indirectly contribute to the benthic insects’ feeding, as more food resources were explored in M+ and a spatial variation of resources intake was observed in this lake, in contrast to the homogeneous feeding in M−. We highlight that macrophytes were responsible for the organic matter build‐up in the sediment, especially at the lake region dominated by these plants, and contributed to increase the deposition of high‐quality amorphous organic matter, which favored taxa in M+ that fed exclusively on this item. The lower diversity of food items exploited in M−, and the Tanypus alga‐based diet in this lake, indicates the low quality of organic resources in its sediment. Although macrophytes were indirectly beneficial for benthic insects’ feeding, we found that this is not an attractive resource for prompt ingestion by most benthic taxa.

Are the patterns of zooplankton community structure different between lakes and reservoirs? A local and regional assessment across tropical ecosystems

Lakes and reservoirs present contrasting differences regarding origin, age and trophic state that may influence their biological communities. In the face of the inevitably rising number of reservoirs worldwide, our objective was to investigate the differences in zooplankton community structure and diversity patterns from 98 tropical shallow lakes and reservoirs (northeast Brazil). We tested the hypothesis that reservoirs have less diverse communities, which could be associated with ecosystem age or high productivity (a typical local pattern). The results show that most reservoirs are eutrophic ecosystems that hold distinct zooplankton communities in comparison with lakes. Despite their higher productivity, reservoirs played an essential role in subsidizing zooplankton diversity as they had higher gamma diversity because of the number of exclusive species, especially for the Rotifera group. The zooplankton density and biomass were also higher in the reservoirs, but this pattern was not associated with higher species dominance. Lakes also played a central role in zooplankton diversity, having a distinct species composition. Jointly, lakes and reservoirs help to maintain the zooplankton species pool at a regional level, suggesting the importance of complementarity in community composition between artificial and natural aquatic ecosystems on large-scale patterns of zooplankton biodiversity.

Crucian Carp (Carassius carassius) Strongly Affect C/N/P Stoichiometry of Suspended Particulate Matter in Shallow Warm Water Eutrophic Lakes

Crucian carp (Carassius carassius) is a key fish species in most Chinese subtropical and tropical shallow lakes. Through sediment feeding, crucian carp could greatly change water turbidity and nutrient levels, as well as the abundance of herbivorous consumers, which may have important influences on seston element stoichiometry. However, so far, experimental studies on this topic are lacking. We conducted a 36-day mesocosm experiment to explore the effects of crucian carp on water physicochemical and biological properties, and C/N/P ratios in suspended particulate matter (SPM) under eutrophic conditions. Our results provided three major findings: (1) Crucian carp resuspended sediments and along with them, reduced light penetration and lower light/total phosphorus (TP) ratios. (2) Crucian carp reduced biomasses of both zooplankton and macrozoobenthos, whereas their effect on phytoplankton was weak, potentially because of resuspension-induced light limitation. (3) Both C/P and N/P ratios in SPM were significant lower in mesocosms with crucian carp than in fish-free controls, which may be attributed to the high contribution of P-rich sediments and low light to nutrient supply caused by fish-induced resuspension. Our results suggest that besides planktivorous fish, benthivore (e.g., crucian carp) in warm shallow waters could also affect pelagic C/N/P stoichiometry via sediment feeding, which may further influence energy transfer efficiency in lake food chain.

Phylogenetic connection among close genera of Aphanizomenonaceae (Cyanobacteria): Amphiheterocytum gen. nov., Cylindrospermopsis and Sphaerospermopsis

Aphanizomenonaceae is a recently proposed family of Cyanobacteria encompassing 12 planktic genera. The formation and distribution of heterocytes and akinetes along the trichome are the main morphological characteristics of this family. In this context, Sphaerospermopsis and Cylindrospermopsis are close related genera both molecularly and morphologically differing by 1) rounded akinetes adjacent to the heterocytes disposed along the trichome in Sphaerospermopsis; 2) apical heterocytes and akinetes separated by a few cells (2–3) in Cylindrospermopsis. Recently, during a survey on cyanobacteria from tropical Brazilian eutrophic lakes, strains with similarities to Sphaerospermopsis and Cylindrospermopsis were isolated. Aiming to elucidate the phylogenetic placement of these strains, we performed morphological, physiological and molecular studies. The Brazilian strains were confirmed to be closely related to Sphaerospermopsis and Cylindrospermopsis, but the phylogeny, based on 16S rRNA gene and 16S–23S rRNA intergenic region, clustered them in a well-defined clade separate from these genera. According to these results, we detailed the morphology, physiology, and phylogeny of the proposed new genus Amphiheterocytum, which showed potential to create blooms in tropical shallow lakes.

Successful restoration of a tropical shallow eutrophic lake: Strong bottom-up but weak top-down effects recorded

Fish manipulation has been used to restore lakes in the temperate zone. Often strong short-term cascading effects have been obtained, but the long term-perspectives are less clear. Fish manipulation methods are far less advanced for warm lakes, and it is debatable whether it is, in fact, possible to create a trophic cascade in warm lakes due to the dominance and high densities of fast-reproducing omnivorous fish. However, removal of benthic feeding fish also reduce disturbance of the sediment, which not only affects the nutrient level but also the concentration of suspended organic and inorganic matter with enhanced water clarity and potentially better growth conditions for submerged macrophytes. We conducted a biomanipulation experiment in one of the basins in Chinese Huizhou West Lake that have remained highly turbid after extensive nutrient loading reduction. Another basin was used as control (control-treatment pairing design). Removal of a substantial amount of plankti-benthivorous fish was followed by planting of submerged macrophytes and stocking of piscivorous fish. We found strong and relatively long-lasting effects of the restoration initiative in the form of substantial improvements in water clarity and major reductions in nutrient concentrations, particularly total phosphorus, phytoplankton and turbidity, while only minor effects were detected for crustacean zooplankton grazers occurring in low densities before as well as after the restoration. Our results add importantly to the existing knowledge of restoration of warm lakes and are strongly relevant, not least in Asia where natural lakes frequently are used extensively for fish production, often involving massive stocking of benthivorous fish. With a growing economy and development of more efficient fish production systems, the interest in restoring lakes is increasing world-wide. We found convincing evidence that fish removal and piscivores stocking combined with transplantation of submerged macrophytes may have significant effects on water clarity in warm shallow lakes even if the zooplankton grazing potential remains low, the latter most likely as a result of high predation on the zooplankton.

The use of polyaluminium chloride as a restoration measure to improve water quality in tropical shallow lakes

Abstract Aim The aim of this study was to evaluate the performance of the coagulant Polyaluminium chloride (PAC) in water quality improvement of six eutrophic shallow lakes in Brazilian semiarid region. Methods We evaluated the effect of PAC in turbidity, humic substances (UV254), total phosphorus and chlorophyll-a concentration through laboratory jar tests. Results The results showed that PAC had a good performance in reducing total phosphorus concentrations and turbidity, with a reduced efficiency in removing chlorophyll-a and humic substances by sedimentation of flocks formed. Conclusions Addition of PAC is a potential tool for water quality improvement of eutrophic shallow lakes in Brazilian semiarid region but its efficiency depends on the pH and particulate and dissolved organic matter concentration in the lake or reservoir water.

Spatial and temporal dynamics of functional groups of phytoplankton in a tropical shallow lake

Abstract Aim To evaluate the structure and dynamics of functional phytoplankton groups (FGs) over a macrophyte cover gradient and their relations with environmental variable in small, shallow clear-water lake. Methods Physical, chemical and phytoplankton analyses were made between August 2014 and June 2015 at three points on the Santa Lucia lake (Paraiba, Brazil). Tukey and Wilcoxon tests were applied to the data followed by CCA and Anova. Results The submerged macrophyte cover and phytoplankton biomass presented high spatial and temporal uniformity. The increase in rainfall induced small variations in functional groups, promoting increase the N group and reduction of the SN. Conclusion The homogeneity in the composition and structure functional groups along macrophyte cover confirm the tendency that in small and shallow lakes communities of limnetic and shoreline zones tend to be similar.

Effects of benthivorous and planktivorous fish on phosphorus cycling, phytoplankton biomass and water transparency of a tropical shallow lake

We investigated the roles of a benthivorous (Prochilodus brevis, Steindachner 1875) and a planktivorous (Oreochromis niloticus, Linnaeus, 1758) fish in translocating phosphorus from the benthic to the pelagic habitat of a tropical eutrophic shallow lake and its impact on phytoplankton biomass and water transparency. We performed two field experiments in 20 mesocosms (6 m3) with a 2 × 2 factorial design. Fish presence/absence was manipulated in combination with the presence/absence of a fish cage (4 m3) that prevented fish accessing the sediment. Benthivorous fish increased total phosphorus and chlorophyll a concentrations and decreased water transparency, but only when they had access to the sediment. Planktivorous fish increased the concentration of chlorophyll a without changing total phosphorus concentrations, whether or not they had access to the sediment. Results suggest that only the benthivorous fish increased phytoplankton biomass by translocating phosphorus from benthic to pelagic habitats. However, the planktivorous fish increased phytoplankton biomass by removing zooplankton and recycling nutrients within the pelagic zone. We conclude that removal of either fish species can improve the water quality of eutrophic shallow lakes in tropical regions, but only the removal of benthivorous fish will reduce the internal loading of phosphorus.

Reproduction of the fish Poeciliopsis gracilis (Cyprinodontiformes: Poeciliidae) in Coatetelco, a tropical shallow lake in Mexico

A reproductive analysis of 1 225 specimens of Poeciliopsis gracilis obtained through monthly samples from Coatetelco, a tropical shallow lake in Central Mexico, was made. There was an evident sexual dimorphism, including a difference in body size at the onset of reproduction. Sex ratio deviated significantly from unity. Monthly variations in gonadosomatic (GSI), hepatosomatic (HSI) indexes and ovarian development stages showed that the spawning season was from July to October, coinciding with the rainy season and phytoplankton biomass increase.