Juan Pablo Pacheco

Environmental Warming in Shallow Lakes: A Review of Potential Changes in Community Structure as Evidenced from Space-for-Time Substitution Approaches

Abstract Shallow lakes, one of the most widespread water bodies in the world landscape, are very sensitive to climate change. Several theories predict changes in community traits, relevant for ecosystem functioning, with higher temperature. The space-for-time substitution approach (SFTS) provides one of the most plausible empirical evaluations for these theories, helping to elucidate the long-term consequences of changes in climate. Here, we reviewed the changes at the community level for the main freshwater taxa and assemblages (i.e. fishes, macroinvertebrates, zooplankton, macrophytes, phytoplankton, periphyton and bacterioplankton), under different climates. We analyzed data obtained from latitudinal and altitudinal gradients and cross-comparison (i.e. SFTS) studies, supplemented by an analysis of published geographically dispersed data for those communities or traits not covered in the SFTS literature. We found only partial empirical evidence supporting the theoretical predictions. The prediction of higher richness at warmer locations was supported for fishes, phytoplankton and periphyton, while the opposite was true for macroinvertebrates and zooplankton. With decreasing latitude, the biomass of cladoceran zooplankton and periphyton and the density of zooplankton and macroinvertebrates declined (opposite for fishes for both biomass and density variables). Fishes and cladoceran zooplankton showed the expected reduction in body size with higher temperature. Life history changes in fish and zooplankton and stronger trophic interactions at intermediate positions in the food web (fish predation on zooplankton and macroinvertebrates) were evident, but also a weaker grazing pressure of zooplankton on phytoplankton occurred with increasing temperatures. The potential impacts of lake productivity, fish predation and other factors, such as salinity, were often stronger than those of temperature itself. Additionally, shallow lakes may shift between alternative states, complicating theoretical predictions of warming effects. SFTS and meta-analyses approaches have their shortcomings, but in combination with experimental and model studies that help reveal mechanisms, the “field situation” is indispensable to understand the potential effects of warming.

Phytoplankton community structure in five subtropical shallow lakes with different trophic status (Uruguay): a morphology-based approach

Phytoplankton abundance and biomass can be explained as a result of spatial and temporal changes in physical and biological variables, and also by the externally imposed or self-generated spatial segregation. In the present study, we analyzed contrasting-season changes in the phytoplankton communities of five subtropical shallow lakes, covering a nutrient gradient from oligo-mesotrophy to hypereutrophy, using a morphologically based functional approach to cluster the species. Six environmental variables accounted for 46% of the total phytoplankton morphological groups variance, i.e., turbidity (Secchi disk), conductivity, total phosphorus, total nitrogen, total zooplankton abundance, and herbivorous meso:microzooplankton density ratio. The differences in resource availability and zooplankton abundance among the systems were related with important changes in phytoplankton composition and structure. Within phytoplankton assemblages, adaptations to improve both light and phosphorus/nitrogen uptake were important in nutrient-poor systems; while grazing-avoidance mechanisms, such as colonial forms or bigger individuals, seemed relatively important only in eutrophic Lake Blanca, where light was not a limiting factor. However, this was not observed in the nutrient-rich Lake Cisne, where low light availability (due to clay resuspension and dark water color) was identified as the main structuring factor. Our results suggest that the composition of phytoplankton morphologically based functional groups appear to reliably describe the trophic sate of the lakes. However, other factors, such as nonbiological turbid condition, or zooplankton composition, may interact rendering interpretations difficult, and therefore, deserve further studies and evaluation.

Seasonal and diel changes in fish activity and potential cascading effects in subtropical shallow lakes with different water transparency

Fish play a key role in the functioning of shallow lakes. Simultaneously, fish are affected by physical in-lake factors, such as temperature and water transparency, with potential changes in their cascading effects on other communities. Here, we analysed the fish community structure and fish activity in four subtropical shallow lakes, varying in trophic state and water transparency, to assess changes promoted by temperature (i.e. summer and winter) and the light regime (i.e. day and night). We used a passive method (gillnets) during the day- and at night-time to detect changes in fish activity, but also sampled the littoral zone (during night) by point sample electrofishing to obtain a better description of the fish assemblage and habitat use. We observed different fish assemblages in the two seasons in all lakes. We captured more fish species and also obtained higher numbers (CPUE with nets) in summer. Contrary to our expectations, the visually oriented Characiformes were the most captured fish regardless of water transparency, at both day-time and night-time. We also found higher fish CPUE at night-time in all lakes. However, the differences between night and day decreased with decreasing transparency, being lower in the least clear lake, Lake Cisne. The nocturnal increase in fish CPUE (including visually oriented species) suggests that darkness serves as a good refuge for fish in shallow subtropical lakes, even at the likely cost of a lower feeding efficiency during the night. The importance of darkness seems to decrease with decreasing water transparency. We also argue that cascading effects of changes in the activity of piscivorous fish (seasonal changes in piscivores CPUE), when omni-planktivorous fish are indeed affected, may eventually reach the zooplankton, but may not be strong enough to reach the phytoplankton, regardless of water transparency.

Stable isotope analysis confirms substantial differences between subtropical and temperate shallow lake food webs

Differences in trophic web structure in otherwise similar ecosystems as a consequence of direct or indirect effects of ambient temperature differences can lead to changes in ecosystem functioning. Based on nitrogen and carbon stable isotope analysis, we compared the food-web structure in a series of subtropical (Uruguay, 30–35°S) and temperate (Denmark, 55–57°N) shallow lakes. The food-web length was on average one trophic position shorter in the subtropical shallow lakes compared with their temperate counterparts. This may reflect the fact that the large majority of subtropical fish species are omnivores (i.e., feed on more than one trophic level) and have a strong degree of feeding niche overlap. The shapes of the food webs of the subtropical lakes (truncated and trapezoidal) suggest that they are fuelled by a combination of different energy pathways. In contrast, temperate lake food webs tended to be more triangular, likely as a result of more simple pathways with a top predator integrating different carbon sources. The effects of such differences on ecosystem functioning and stability, and the connection with ambient temperature as a major underlying factor, are, however, still incipiently known.

What can resting egg banks tell about cladoceran diversity in a shallow subtropical lake

Dormant stages (“resting eggs”) produced by cladocerans can persist for long periods of time in sediments and restore populations once the environmental conditions become favorable again. Lake Blanca, a subtropical shallow eutrophic lake, hosts a cladoceran community dominated by small-sized species. Previous studies on zooplankton resting eggs suggested that the cladoceran genera Daphnia and Simocephalus were present, but they had never been found before in water samples. In the present study, we compared a biweekly active community sampling with the resting egg bank (passive cladoceran community) from littoral and pelagic zones. Moreover, we tested the amount of samples required to have a representative reconstruction of the diversity in both compartments (water and sediment). Lake Blanca showed a relatively high cladoceran species richness (24) in the water column, with rapid temporal replacement. Several species were present in water samples during short temporal windows; therefore, to detect these species a strong sampling effort in terms of temporal frequency and spatial distribution was required. Contrary to our expectations, resting egg community showed a lower diversity than the active community; however, we demonstrated that the analysis of resting egg bank composition can help detect general community structure patterns.

Agriculture and elevation are the main factors for Pampasic stream habitat and water quality

Streams of the Pampasic plain in Southeastern South America are ecosystems affected by both water pollution and habitat alteration mainly due to agricultural activity. Water quality is influenced by the quality of habitats and both depend on land use and watershed morphology. The objective of this study was to determine the relationship between the variables of four factors: (1) the morphology of the watershed, (2) land use in the watershed, (3) river habitat, and (4) water quality of wadeable streams in Uruguay, as well as to determine the most representative variables to quantify such factors. We studied 28 watersheds grouped into three ecoregions and four principal activities, which generated seven zones with three to five streams each. Correlations between the variables of each factor allowed reducing the total number of variables from 57 to 32 to perform principal component analyses (PCA) by factor, reducing the number of variables to 18 for a general PCA. The first component was associated with water quality and elevation. The second was associated with the stream and watershed size, the third with habitat quality, and the fourth to the use of neighboring soils and objects in the channel. Our results indicate that agricultural intensity and elevation are the main factors associated with the habitat and water quality of these lowland streams. These factors must be especially considered in the development of water quality monitoring programs.

Factores ambientales controladores del fitoplancton con énfasis en las cianobacterias potencialmente tóxicas en un lago somero utilizado como fuente de agua para potabilización: Laguna del Sauce, Maldonado, Uruguay

Laguna del Sauce, principal fuente de agua para potabilizacion en Maldonado, Uruguay, ha registrado recurrentes eventos de floraciones de cianobacterias potencialmente toxicas, que comprometen su uso como tal. Esto ha denotado la necesidad de un mayor conocimiento de los factores ambientales asociados a la composicion fitoplanctonica y, en especial, a la ocurrencia de floraciones de cianobacterias. Mediante muestreos semanales estivales (2015-2016) de variables fisicoquimicas, nutrientes y fitoplancton, en seis sitios de Laguna del Sauce, se analizaron los factores ambientales asociados a la biomasa y composicion fitoplanctonica, particularmente de cianobacterias. La alta variabilidad temporal y espacial observada en la composicion fitoplanctonica presento alternancia de floraciones de cianobacterias: Dolichospermum crassum, Cuspidothrix issatschenkoii, Microcystis aeruginosa, Microcystis panniformis y Sphaerocavum brasiliense. La turbiedad fue clave para regular la biomasa fitoplanctonica, mientras que la relacion Nitrogeno total (NT):Ortofosfato (PO4) se asocio a la presencia diferencial de especies de cianobacterias. El analisis de las relaciones entre estos nutrientes puede contribuir a anticipar la ocurrencia de floraciones de diferentes especies de cianobacterias en este ecosistema, posibilitando la implementacion de medidas de gestion orientadas a disminuir sus efectos adversos y, en particular, a asegurar el proceso de potabilizacion.

Dinámica ecosistémica y calidad de agua: estrategias de monitoreo para la gestión de servicios asociados a Laguna del Sauce (Maldonado, Uruguay)

La eutrofizacion constituye uno de los principales problemas de los ecosistemas acuaticos continentales. Una de las consecuencias mas conocidas de este proceso son las floraciones de algas y cianobacterias, que interfieren en el adecuado funcionamiento de los ecosistemas acuaticos y en los servicios que estos brindan. En consecuencia, resulta relevante el desarrollo de estrategias de monitoreo que permitan conocer el funcionamiento de los sistemas, para anticipar la evolucion temporal del fitoplancton y sus efectos en la calidad del agua y aportar informacion trascendente en los procesos de toma decision en diferentes escalas espacio-temporales y que atienden problematicas diversas. Este trabajo pretende contribuir a ese desarrollo, sistematizando la informacion hoy disponible en lo referente a monitoreo y analisis de los procesos subyacentes a la dinamica del fitoplancton en Laguna del Sauce (Maldonado, Uruguay). Se identifican los principales avances, dificultades y desafios, enfatizando la necesidad de la integracion de la informacion y de los procesos de analisis generados por diversas instituciones, asi como el planteo de estrategias que promuevan la permanencia en el tiempo de estas practicas.