Luigi Naselli-Flores

Ecological impacts of global warming and water abstraction on lakes and reservoirs due to changes in water level and related changes in salinity

According to the Intergovernmental Panel on Climate Change report released in September 2014, unprecedented changes in temperature and precipitation patterns have been recorded globally in recent decades and further change is predicted to occur in the near future, mainly as the result of human activity. In particular, projections show that the Mediterranean climate zone will be markedly affected with significant implications for lake water levels and salinity. This may be exacerbated by increased demands for irrigation water. Based on long-term data from seven lakes and reservoirs covering a geographical gradient of 52° of latitudes and a literature review, we discuss how changes in water level and salinity related to climate change and water abstraction affect the ecosystem structure, function, biodiversity and ecological state of lakes and reservoirs. We discuss mitigation measures to counteract the negative effects on ecological status that are likely to result from changes in climate and water abstraction practices. Finally, we highlight research required to improve knowledge of the impacts of anthropogenically induced changes on lake water level and consequent changes in salinity.

Water-Level Fluctuations in Mediterranean Reservoirs: Setting a Dewatering Threshold as a Management Tool to Improve Water Quality

Water-level fluctuations, often linked to seasonal climatic trends, are a natural phenomenon which occur in almost all aquatic ecosystems. In some climatic regions, as the Mediterranean one, they are particularly wide due to the occurrence of two well separated periods: the rainy winter and the almost completely dry summer. Precipitation is concentrated in the first period, whereas in the second strong evaporation losses take place. According to these climatic features, and to ensure a continuous supply of water throughout the year, man-made lakes store water during winter and are subjected to dewatering during summer to compensate the lack of precipitation. These ecosystems are thus characterised by rather wide water level fluctuations which were observed to transform them from potentially warm monomictic lakes into polymictic or atelomictic ones. These changes deeply affect the biological structure and the functions of the water bodies impairing the response of some ecosystem properties, as resilience and resistance, since the impacts are immense enough to move the systems out of their homeostatic plateau of, respectively, deep or shallow lakes. In order to understand to what extent a reservoir can be “emptied” without changing its ecosystemic identity (deep or shallow lake sensu Padisák & Reynolds, 2003) and to set a “dewatering threshold”, the results from two different hydrological years, one with a dewatering so intense as to disrupt thermal stratification in midsummer, and the other one with water enough to allow the maintenance of the reservoir’s thermal structure throughout the summer, are compared. Former investigations have shown that the persistence of thermal stratification has a positive value in Sicilian reservoirs: a notable decrease in total phytoplankton biomass and in the relative occurrence of cyanoprokaryotes was observed in the high-level year with a stable thermal stratification. Although the solving of the external load problems causing eutrophication phenomena remain the main task to improve the water quality of this Mediterranean island, a management procedure, based on the maintaining of the ecosystem within its homeostatic plateau through the setting of a dewatering threshold, is suggested.

Shape and size in phytoplankton ecology: do they matter?

This paper summarises the outcomes of the 14th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP). The authors mostly addressed their contributions on the following topics: morphological and morpho-functional descriptors of phytoplankton, size and shape structure of phytoplankton related to different kinds of environmental variables and the role of morphological and physiological plasticity of phytoplankton in maintaining the (apparently) same populations under different environmental conditions. Case studies from different kinds of aquatic environments (deep and shallow lakes, reservoirs with different age, purpose and trophic state, floodplain wetlands mostly in the temperate region but also from subtropical and tropical ones) have shown that similar environmental forcing calls for similar morpho-functional properties even though the corresponding associations can be markedly different on species level.

Equilibrium/steady-state concept in phytoplankton ecology

This paper summarises the outcomes of the 13th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP). The authors mostly addressed their contributions on the following topics: the effect of trophic state on the attainment of a steady-state; the establishment of equilibria in deep and shallow lakes; the role of spatial heterogeneity, disturbance, and stress in the establishment of equilibrium assemblages; the mechanisms leading to the steady state; the frequency and longevity of equilibrium phases, and the role of morphological and physiological plasticity of phytoplankton in maintaining the (apparently) same populations under different environmental conditions. The composition of steady-state assemblages is compared to that of phytoplankton functional groups (coda). Those functional associations recognised as steady-state assemblages appear to be strongly K-determined in many instances.

Phytoplankton assemblages in twenty-one Sicilian reservoirs: relationships between species composition and environmental factors

Data collected in a limnological survey, carried out between 1987 and 1988 on 21 Sicilian reservoirs of varying trophic state, were ordinated using CANOCO 3.1 to generalise the way in which the structure of phytoplankton assemblage is conditioned by both physical and chemical variables. The results showed that in these man-made lakes, characterised by conspicuous water-level fluctuations, the annual and interannual variability in the abundance and composition of phytoplankton may be strongly influenced by their peculiar hydraulic regimes rather than by nutrient availability. In particular, it was highlighted that, from the early summer, water abstraction often leads to increased circulation and to the deepening of the mixed layer. In this way, an increase of the ratio of mixing depth to euphotic depth is forced, with the result that phytoplankton cells experience longer periods in darkness as they are carried through the mixed layer. Phytoplankton assemblages change in species composition in response to the environmental variation. Both the raising of the trophic state, with an increase in phytoplankton biomass and a decrease in transparency, and the intensified abstraction enhance the role of light availability in promoting the development of specific phytoplankton assemblages adapted to the modified physical environment. Light climate is an important influence on the species structure of the phytoplankton, especially in the higher part of the trophic gradient. In contrast, the influence of nutrients on the structure of the assemblages appears to be higher in the lower part of the trophic spectrum or in those environments characterised by a higher hydrological stability during the year.

Man-made lakes in Mediterranean semi-arid climate: the strange case of Dr Deep Lake and Mr Shallow Lake

The lack of any protection against eutrophication is progressively compromising the water quality of Sicilian reservoirs. These water bodies provide the population with an adequate supply of drinking water and support most of the irrigation requirements of local agriculture. Moreover, they respond to the Mediterranean climate, with sequential, seasonally predictable events of flooding and drying and whose intensity varies markedly between years. As a consequence of summer drought and the policy to meet water demand throughout the season, the reservoirs experience massive dewatering, resulting in a reduction in the spring storage volume of 90%. Thus, they start the hydrological season as water bodies deep enough to form stable stratification but a thermocline has generally disappeared by the middle of summer and, as a consequence of the intense drawdown, they are transformed into shallow water bodies. This dual behaviour, driven by a progressive and conspicuous depth decrease, interferes with nutrient and phytoplankton dynamics, enhancing eutrophication phenomena and contributing to the selection of planktic cyanobacteria that are potentially dangerous to human health.

Phytoplankton dynamics and structure: a comparative analysis in natural and man-made water bodies of different trophic state

Previous investigations on Sicilian man made lakes suggested that physical factors, along with the specific morphology and hydrology of the water body, are important in selecting phytoplankton species. In particular, the variations of the zmix/zeu ratio due to the operational procedure to which reservoirs are generally subject were recognised as a trigger allowing the assemblage shift. To investigate if these variations may be considered analogous to those occurring in natural lakes as trophic state and phytoplankton biomass increase, causing a transparency decrease and a contraction of the euphotic depth, phytoplankton were collected in two natural water bodies, one mesotrophic (Lake Biviere di Cesarò) the other eutrophic (Lake Soprano), and compared with those collected in two reservoirs with analogous trophic characteristics (Lake Rosamarina, mesotrophic and Lake Arancio, eutrophic). Particular attention was paid to the dynamics of two key groups: Cyanophytes and chlorophytes. In all four water bodies, transparency mainly depended on chlorophyll level. Annual average value of phytoplankton biomass in the mesotrophic environments was below 2.0 mg l−1, whereas in the eutrophic systems it was well above 10 mg l−1. All water bodies showed the presence of cyanophytes (e.g. Anabaena spp., Anabaenopsis spp., Microcystis spp., Planktothrix spp.) and chlorophytes (e.g. Chlamydomonas spp., Botryococcus spp., Oocystis spp., Scenedesmus spp., Pediastrum spp.), but their relative proportions and body size dimensions were different. In particular, small colonial chlorophytes and large-colony forming cyanophytes were most common in the most eutrophic water bodies, whereas larger colonies of green algae in those with a lower trophic state. The results showed that, under the same climatic conditions, autogenic (increase of biomass, decrease in light penetration and euphotic depth) and allogenic (use of the stored waters, anticipated breaking of the thermocline, increase of the mixing depth) processes may shift the structure of phytoplankton assemblage in the same direction even though the quantity of biomass remains linked to nutrient availability.

How do freshwater organisms cross the “dry ocean”? A review on passive dispersal and colonization processes with a special focus on temporary ponds

Lakes and ponds are scattered on Earth’s surface as islands in the ocean. The organisms inhabiting these ecosystems have thus developed strategies to pass the barrier represented by the surrounding land, to disperse and to colonize new environments. The evidences of a high potential for passive long-range dispersal of organisms producing resting stages inspired the idea that there were no real barriers to their actual dispersal, and that their distribution was only limited by the ecological characteristics of the available habitats. The development of genetic techniques allowed to criticize this view and revealed the existence of a more complex and diverse biological scenario governed by an assortment of historical and ecological factors. In this paper, we review the literature related to the passive dispersal of organisms producing resting stages among inland lentic ecosystems, with special emphasis to temporary ponds, which represent “isolated” ecosystems both in space and in time, and are characterized by high levels of biological diversity. The existence of a sharp decoupling between “dispersal potential” and “actual establishment rates” is stressed, thus urging a definitive overcome of the so-called “Everything is Everywhere” hypothesis in order to gain a proper understanding of the biogeography and ecology of inland water organisms.

Pluriannual morphological variability of phytoplankton in a highly productive Mediterranean reservoir (Lake Arancio, Southwestern Sicily)

The progressive decrease in water quality experienced by the Mediterranean, nutrient rich reservoir, Lake Arancio (Southwestern Sicily, Italy), has been accompanied by modifications in both phytoplankton shape and size. Since nutrient concentrations are always far from being limiting, in the present paper attention was focussed on the effects exerted by physical constraints (e.g., mixing, light availability) on the morphological variation observed in phytoplankton. The surface–volume ratio and its product with the maximal linear dimension were chosen as morphological descriptors to trace the trajectories followed by the dominant morphotypes in Lake Arancio. One of these descriptors was compared to the mixing depth—euphotic depth ratio, considered as a good descriptor of the underwater light climate. The strong influence exerted by light transmission and mixing regime on phytoplankton morphological traits and thus on its composition in this environment confirms the paramount importance of both light transmission and the peculiar hydrological regime to which reservoirs are subjected, especially in Mediterranean climate, and points out the need to integrate hydrology into ecological studies and management procedures.

Distribution and seasonal dynamics of Cryptomonads in Sicilian water bodies

Several species with a relevant ecological importance belong to Cryptophyta. Nevertheless, species-level identification from microscopic observations is problematic, lacking recent taxonomic keys. In this study we report our observations on distribution and seasonal dynamics of Cryptomonads in 33 Sicilian water bodies, as well as the main taxonomical problems we encountered. Species of the genera Cryptomonas and Plagioselmis are the most common in the examined water bodies. Their biomass seasonal trends usually show a peak in late winter and early spring. In advanced spring, and also in summer, due to the higher grazing pressure, the Cryptomonads biomass reaches its lowest values. Moreover, in a small, eutrophic, temporary pond (Santa Rosalia), without grazing pressure in spring, the Cryptomonads bloomed in summer. From our results, we propose to focus the attention on the sensitiveness of these organisms to filter feeding to explain their seasonal dynamics.