Susana Romo

State of the art in the functioning of shallow Mediterranean lakes: workshop conclusions

Studies on shallow lakes from the north temperate zone show that they alternate between clear and turbid water states in response to control factors. However, the ecology of semiarid to arid shallow Mediterranean lakes is less explored. Hydrological effects (e.g. water level fluctuations, water residence time) on major ions and nutrient dynamics and processes, and ecology of submerged macrophytes appear to have a crucial role for food webs in shallow Mediterranean lakes. Nutrient control may be of greater priority in eutrophicated warm shallow lakes than in similar lakes at higher latitudes. This will be relevant for the implementation of the European Water Framework Directive, and conservation and management of these ecosystems. Strong trophic cascading effects of fish resulting from dominance of omnivorous and benthivorous fish species, whose diversity is usually high, together with frequent spawning and absence of efficient piscivores, seem to be the reason for the lack of large-bodied grazers that could control phytoplankton. However, such effects may vary within the region depending on fish distribution and community. These factors need elaboration in order to allow shallow lake ecologists and managers to develop better restoration strategies for eutrophicated shallow Mediterranean lakes. Consequently, modifications for the implementation of the European Water Framework Directive for determining ecological status in shallow Mediterranean lakes appear to be necessary. Furthermore, the implications of climate warming may be even more challenging than in high latitude lakes since shallow lakes in the Mediterranean region are among the most sensitive to extreme climate changes. There is an urgent need for data on the ecology of shallow lakes in the region. An appeal is made for international cooperation, development of large-scale research and information exchange to facilitate this and a web-based discussion list has been implemented.

Population dynamics and ecology of subdominant phytoplankton species in a shallow hypertrophic lake (Albufera of Valencia, Spain)

Long-term population dynamics and ecology of the abundant but non-dominant phytoplankton species of the shallow hypertrophic lake the Albufera of Valencia (Spain) are described for the period 1980–1988. The lake is used as a reservoir for the surrounding ricefield cultivation. It is continuously dominated by three filamentous blue-green algae, Planktothrix agardhii, Pseudanabaena galeata and Geitlerinema sp. Horizontal differences of the phytoplankton were less important than annual and seasonal variations. An annual increasing trend was observed for Planktolyngbya subtilis, Planktolyngbya contorta, Cylindrospermopsis raciborskii, Microcystis incerta, Nitzschia palea var. tenuirostris and Rhodomonas lacustris var. nannoplanctica, whereas Anabaenopsis elenkinii, Scenedesmus acuminatus, Scenedesmus quadricauda and Cyclotella meneghiniana showed an opposite trend. This pattern seems related to the increase of nitrogen and phosphorus loading in the lake and certain hydrological changes occurred in the latter years of the study. Seasonal pattern of the subdominant species showed the presence of Cyclotella meneghiniana, Scenedesmus species and Chlamydomonas spp. in spring, during moderate water renovation rates and phosphate availability. Periodicity shifted to blue-green algae and Nitzschia species in summer and early autumn, during reduced phosphate levels and initial low but later high water renewal. Autumn and winter species, such as Monoraphidium contortum, Nitzschia gracilis, Rhodomonas lacustris var. nannoplanctica and Cryptomonas erosa, were mainly related to light intensity and temperature. Although the lake has a typical hypertrophic phytoplankton assemblage, the general seasonal variation of the species is similar to that often described in temperate lakes. Diatom species composition differs to that cited in other shallow hypertrophic lakes of Europe.

Predation and competition effects on the size diversity of aquatic communities

Body size has been widely recognised as a key factor determining community structure in ecosystems. We analysed size diversity patterns of phytoplankton, zooplankton and fish assemblages in 13 data sets from freshwater and marine sites with the aim to assess whether there is a general trend in the effect of predation and resource competition on body size distribution across a wide range of aquatic ecosystems. We used size diversity as a measure of the shape of size distribution. Size diversity was computed based on the Shannon-Wiener diversity expression, adapted to a continuous variable, i.e. as body size. Our results show that greater predation pressure was associated with reduced size diversity of prey at all trophic levels. In contrast, competition effects depended on the trophic level considered. At upper trophic levels (zooplankton and fish), size distributions were more diverse when potential resource availability was low, suggesting that competitive interactions for resources promote diversification of aquatic communities by size. This pattern was not found for phytoplankton size distributions where size diversity mostly increased with low zooplankton grazing and increasing nutrient availability. Relationships we found were weak, indicating that predation and competition are not the only determinants of size distribution. Our results suggest that predation pressure leads to accumulation of organisms in the less predated sizes, while resource competition tends to favour a wider size distribution.

Epiphyton, phytoplankton and macrophyte ecology in a shallow lake under in situ experimental conditions

The present experimental research reports the responses of phytoplankton, epiphyton and charophytes in a shallow characean Mediterranean lake under different nutrient levels (nitrogen and phosphorus) and planktivorous fish densities in two in situ mesocosm experiments. How these factors affected biomass, dominance and replacement of these primary producers were quantified and detailed. Diversity and the structure of phytoplankton and epiphyton were also examined, as well as algal selectivity between habitats determined by daily recruitment of the epiphyton in the water column under different nutrient and grazing conditions. Our findings indicated that epiphyton biomass underwent a logarithmic increase at lower nutrient levels than phytoplankton, which increased linearly and dominated with total phosphorus above 0.35 mg L -1 . Both phytoplankton and epiphyton correlated negatively to submerged plant biomass, while phosphorus levels above 0.1-0.2 mg L -1 prevented the co-existence of charophytes, epiphyton and phytoplankton. Charophyte biomass was highest with total phosphorus below 0.06 mg L -1 when phytoplankton chlorophyll-a was below 35 μg L -1 and epiphyton chlorophyll-a below 200 μg g -1 plant dry mass. Planktivorous fish densities exerted a minimal effect on the biomass and composition of epiphyton and phytoplankton and on the biomass of charophytes. Diversity of planktonic and epiphytic algae significantly decreased with nutrient additions. Epiphyton and phytoplankton shared 85-88% of the species composition, suggesting that most of them were tychoplanktonic. Daily algal recruitment from the periphyton in the water column was higher at intermediate nutrient levels of 0.6 mg L -1 N and 0.06 mg L -1 P, especially for filamentous cyanobacteria.

Multivariate analysis of phytoplankton and related environmental factors, in a shallow hypertrophic lake

Data on some relevant environmental variables and phytoplankton species composition, collected from the hypertrophic shallow lake Albufera of Valencia (Spain) during 1980–88, were examined using Redundancy Analysis (RDA). The hydrological cycle of the lake is manipulated for rice cultivation in the area. Seasonality and the particular hydrological cycle of the lake were the principal factors influencing long-term phytoplankton dynamics. Annual or horizontal differences were less important than the seasonal factor. However, a trend of phosphate increase and underwater illumination decrease was observed between 1980 and 1988. These changes might be related to some species year-to-year variations, although in general interannual phytoplankton changes were scarce. Spatial phytoplankton differences were much smaller than physical and chemical differences, which were mainly related to loading and residence times at the different sampling zones. Shallowness, hypertrophy and the regular hydrological cycle of the Albufera for rice yield, seem to contribute to the maintenance of an almost stable and homogeneous algal community, mainly composed of filamentous cyanophytes. RDA analysis has proved to be an efficient method in yielding valuable information on phytoplankton-environment interactions and trends over a long series of data. It seems also a feasible technique to monitor the results of lake management and restoration in the future.

Long-term phytoplankton changes in a shallow hypertrophic lake, Albufera of Valencia (Spain)

A long-term phytoplankton study was carried out in the Albufera of Valencia, a shallow hypertrophic lake (surface area 21 km2, mean depth 1 m, total inorganic nitrogen load 155 g m-2 y-1, total inorganic phosphate load 15 g m-2 y-1) from 1980 to 1988. The lake functions as a reservoir for the surrounding rice cultivation. From 1940s to 1988, its phytoplankton assemblage has been altered from a mesotrophic to a hypertrophic character, as consequence of the increasing pollution. For 1980–88, annual variations in the phytoplankton were less pronounced than seasonal changes. The hypertrophic and morphometric features of the lake favoured the stability of the phytoplankton assemblage and chlorophyll a levels during the study period. Seasonal and horizontal distribution of the total phytoplankton abundance and biomass were highly influenced by the hydrological cycle of the lagoon. Compared with other shallow nutrient rich lakes, the Albufera of Valencia is similar to the shallow hypertrophic lakes of the Netherlands.

Taxonomic investigation using DNA fingerprinting in Geitlerinema species (Oscillatoriales, Cyanobacteria)

The taxonomic study of 14 strains of Geitlerinema amphibium (Ag. ex Gom.) Anagnostidis and Geitlerinema unigranulatum (R.N. Singh) Komárek and Azevedo, coming from several localities was undertaken. Use was made of morphological data and molecular data were obtained by means of the DNA fingerprinting technique using highly iterated palindrome (HIP1) sequences. The employed morphological characteristics were those used for species taxonomic identification belonging to the Geitlerinema genus, namely, cell dimensions, shape of the apical cell, motility, number and localization of cyanophycin granules in the cell. The two species revealed as polymorphic were discriminated only by means of the average cellular diameters. In spite of this, minima and maxima values of the cellular diameters overlapped. It was found from molecular analysis that a high genetic diversity and the formation of two clusters consisted of G. amphibium and G. unigranulatum, plus a sole strain keeping itself isolated from the remaining. Also, these clusters were not related to the geographic location; they encompassed strains from water bodies distant from each other by as much as 3500 km, or Brazilian and Spanish strains. Molecular and morphological data support the possibility that G. unigranulatum could be considered a synonym for G. amphibium. HIP1 fingerprinting is a powerful tool for the study of genetic of cyanobacteria closely related taxa. This study points to the necessity of using other than morphological data in the taxonomic revision of cyanobacteria, as well as in the proposition of new taxons.

Temporal changes of cyanobacteria in the largest coastal Spanish Lake

In this work, we will present long-term changes of cyanobacteria in the largest coastal, shallow, Spanish freshwater lake (the Albufera of Valencia). The lake is used as a reservoir for rice cultivation in the sorrounded lake area. The Albufera lake was in a mesotrophic state during the first half of the 20 th century and at the end of 1960s, due to eutrophication, it rapidly turned into an eutrophic state with loss of submerged macrophytes and general biodiversity in the plankton, benthos and fish populations, that remains until now. Since the beginning of the century, cyanobacteria were represented in the phytoplankton of the lake, but from 1970s to 2000 became the dominant group. A total of 26 cyanobacteria species have been described between 1942 and 2000. Morphological and phenological features of the species are shortly reported in this study. Since 1991, a restoration plan of the lake started by sewage water diversion, that has reduced to half planktic chlorophyll a concentrations, although hypertrophic levels persist. Some alternative clear water phases appeared in which cyanobacteria contribution was reduced from 50-90% to less 5%. After nutrient diversion also a shift in cyanobacteria composition was observed. Large oligophotic filamentous species (mainly Plankiothrix agardhii) were replaced by slender filamentous species (mainly Pseudanabaena galeata, Planktolyngbya contorta, Pl. limnetica and Jaaginema cf metaphyticum) and chroococcals (Chroococcus dispersus, Ch. minutus and Aphanocapsa incerta). Water stability, filament resuspension from the sediment, turbidity and benthic-planktivorous fish, were mechanisms stabilizing cyanobacteria dominance in the lake, while high flushing, nutrient depletion and increase of underwater light destabilized or prompted changes in the cyanobacteria community.

Preservation of filamentous cyanobacteria cultures (Pseudanabaena galeata) Böcher and Geitlerinema amphibium (Ag. ex Gom.) Anagn. under low temperatures

Abstract A method for freezing preservation of the filamentous cyanobacteria Pseudanabaena galeata Bocher and Geitlerinema amphibium (Ag. ex Gom.) Anagn. at −30°C and −80°C was examined. Four freezing procedures, two different cryoprotectors and two growth states were assayed. The use of 5% v/v dimethyl sulphoxide as cryoprotectant produced better results than methanol 1.5 M. In general, recoveries over all the storage temperatures and freezing procedures tested were higher in G. amphibium than in P. galeata , which was more sensitive to intercellular trichome breakage. The storage of P. galeata and G. amphibium cultures in stationary and logarithmic growth phase respectively, with 5% v/v dimethyl sulphoxide at −30°C or −80°C, proved to be a suitable conservation technique for these cyanobacteria.

Nutrients and carbon in some Mediterranean dune ponds

Nutrients and carbon were studied in the temporary and permanent dune ponds of one of the few restored Mediterranean dune systems. The results indicate that small, low nutrient dune ponds can have a relevant role for carbon storage and processing, especially in the limestone areas. Despite the low percentage of organic matter in the sediment and water nutrient concentrations, most of the ponds had supersaturated carbon dioxide partial pressure levels with concentrations in half of them greater than the mean global value for lakes. Concentrations gradually decreased from old to newly restored ponds and with the presence of submerged plants. Carbon content was greater in temporary ponds. DOC was half of the mean global value for lakes. Particulate forms of nutrients and dissolved inorganic carbon predominated. Organic matter was a key variable for storage of nutrients and carbon in the sediment. Flooding of the ponds significantly reduced total organic carbon in the sediment by about 60%, but did not change significantly the concentrations of nitrogen and phosphorus. However, about 25% of organic matter and 37% of nitrogen accumulated in the sediment during consecutive summer droughts. Benthic algae incorporated part of the sediment nutrient pool after flooding.