Flavio Corradini

Water residence time as a driving force of zooplankton structure and succession

Abstract.We studied zooplankton dynamics in a groundwater-fed, montane lake during four consecutive years and assessed the importance of water residence time for zooplankton dynamics. Crustacean abundance and biomass were significantly correlated with water residence time and temperature, but showed no significant correlation with phytoplankton biovolume. We hypothesised that temperature depended on water residence time (τ), and therefore we further investigated the functional relationship of crustacean dominance with the latter by logistic regression analysis. Water residence time values above a threshold value (τ = 193 days) determined crustacean biomass dominance while values below determined rotiferan dominance. Our results indicated that water residence time was an important factor structuring zooplankton succession in this lake that showed large fluctuations of τ values (median 263 days; range 23 – 786 days for the four year period) compared to other lakes. We suggest that crustacean biomass was directly controlled through water residence time as found for riverine systems, whereas rotifer biomass was controlled through exploitative competition with crustaceans for phytoplankton. The importance of water residence time may have been underestimated in lakes when explaining zooplankton community structure and succession, because studies usually focus on other factors such as temperature, predation, or food limitation.

Weather-driven ecology of planktonic diatoms in Lake Tovel (Trentino, Italy)

The seasonal development of planktonic diatoms in Lake Tovel has been studied from 2002 to 2004 within the SALTO research project. The objective was to investigate the role of weather conditions and lake hydrology in regulating the development of Cyclotella and Fragilaria functional species groups. The different responses of the two species to the environmental variability and their reciprocal relationships were considered. Cyclotella showed a weak relationship with the selected environmental variables and its seasonal development was difficult to interpret, also in relation to its opportunistic behaviour. On the contrary, Fragilaria complex showed a prompt response to environmental variability and its seasonal development appeared to be mainly regulated by hydrological conditions, through nitrate nitrogen and, secondarily, by silica concentrations. Water temperature and thermal stability affected only Fragilaria species, while different nutrient requirements allowed Cyclotella species to grow under conditions of limited resources for Fragilaria. Since Lake Tovel has scarce internal nutrient reserves and its hydrology is strongly affected by precipitation both in winter and summer, diatom phenology seems to be indirectly regulated by weather conditions.

Long-term trends in species composition and diurnal migration of dinoflagellates in Lake Tovel (Trentino, Italy)

Lake Tovel, an oligotrophic mountain lake (Trentino, N. Italy) is famous for its past bright red summer blooms of the dinoflagellate Glenodinium sanguineum Marchesoni. In 1938 and 1941 Baldi studied the lake and bloom phenomenon in detail. In particular, he found that the dinoflagellate showed high morphological plasticity and that it was present in the whole lake, but concentrated in the Red Bay, where it exhibited an active vertical migration. In 2001 the presence of G. sanguineum in the whole lake, with higher densities in the Red Bay could be confirmed. However the cell densities found in the Red Bay are up to 103 lower than those reported by Baldi. During two diurnal studies in August 2001 almost identical diurnal depth variations of maximum density of G. sanguineum were found, likely to be related to positive phototaxis. Although in the Red Bay the patterns of vertical variations were less clear, potential influential factors (wind, sun, temperature) are analysed and discussed. Dinoflagellate species composition from this and earlier studies show an increase in the number of taxa recorded from the lake in recent years and a shift in species composition possibly related to changes in the trophic status of the lake.

Reinterpretation of the dinoflagellate ‘Glenodinium sanguineum’ in the reddening of Lake Tovel, Italian Alps

Abstract Lake Tovel in the Italian Alps is famous for its blood-red water during summer, caused by a dinoflagellate named Glenodinium sanguineum. The red colour has been largely absent since 1964 and a project aimed at understanding the underlying cause of the colour change was begun in 2000. It appears that there are three dinoflagellates in the lake that morphologically somewhat resemble ‘G. sanguineum’. One of these agrees with the ‘red form’ of G. sanguineum studied in the detailed work of Baldi (1941), and is now very scarce in the plankton. The other agrees with Baldis ‘green form’ and now dominates the plankton. Transmission electron microscopy has demonstrated that the third taxon is identical to what Dodge et al. (1987) identified as G. sanguineum from Lake Tovel. It did not develop a red colour under any of the growth conditions used in our experiments. The red form is very similar to Woloszynskia coronata, but differs in cyst morphology. It is shown that the red and green forms of G. sanguineum sensu Baldi are clearly distinct species. Reduction in nutrient loads entering the lake subsequent to changes in animal husbandry practices in the lakes catchment occurred around 1964. This apparently tipped the balance between the three species of dinoflagellates, resulting in the near disappearance of the red form from the plankton and concomitant disappearance of the red colour from the lake.

Influence of environmental factors on diatoms and dinoflagellates in Lake Tovel (Trentino, Italy)

Lake Tovel is an oligotrophic mountain lake ( 1177 m a.s.l.) located in the Brenta Dolomites (Central Alps). The lake has become famous since the beginning of the 20th century because o f red blooms o f the dinoflagellate Glenodinium sanguineum Marchesoni. The reddening occurred in the small and shallow (max. depth = ca. 4.5 m) SW lake basin called Red bay, usually during the central hours of the day between July and September (BALDI 1941, MARCHESONI 1959). The phenomenon sudden1y ceased after 1964. Since then, numerous investigations have been carried out at the 1ake to understand the red bloom dynamics (e.g. PAGANELLI 1992, CANTONATI et al. 2003). The present investigation was conducted within the SALTO project (2001-2004) and represents the last contribution to these studies. The water regime ofLake Tovel has a very dynamic relationship with the morpho1ogy of its large calcareous catchment: retention time is very short after ice thaw (PAGANELLI 1992), and the lake level decreases beneath the outlet level during dry periods because of submerged outlets (FERRETTI & BORSATO, in press). We investigate the influence o f physical and chemical variables on abundance and distribution of two key phytoplankton divisions for Lake Tovel in summer 2002 and 2003: (a) the diatoms, currently the dominant group in sununer (relative abundance up to ca. 70% o f algae ); (b) the dinoflagellates, responsible for red blooms and still abundant in sununer. The study focuses on the period May-October, corresponding to the fastest hydrological dynamics, and on Red Bay, si te o f past reddening. Despite its shallow depth, Red Bay shows a pronounced sununer stratification due to the input of numerous submerged springs located along the shore, which maintain water temperature o f the deepest layers ( ca. l m above the bottom) at around 5 °C (CORRADINI et al. in prep.).

The importance of hydraulic conditions in determining ecological equilibrium in Lake Tovel, Italy

Oligotrophic Lake Tovel is famous for its past bloodred Glenodinium sanguineum summer algal blooms in the Red Bay. A unique feature ofthe lake is its dynamic water regime; BALDI (1941) considered the Iake to be fluvial-lacustrine. The area s catchment is pseudokarstic, and the main surface inflow disappears about I km from the lake into a porous aquifer feeding the perilittoral springs in the Red Bay (Fig.l ). High catchment-to-lake-area ratio, together with high mean elevation, contributes to fast water renewal in spring and early summer concomitant with snow melt. The aquifers volume (> 50% lake volume), guarantees a continuous inflow of cold (-5 0 C) water to the lake; however, in very dry periods inflow does not compensate outflow, causing a decrease in lake Ievel, particularly evident in the Red Bay (FERRETTI & BORSATO 2004).

Anthropogenically induced phytoplankton blooms in Lake Serraia, N. Italy

Lake Serraia is a small dimictic lake in the province ofTrento, N. ltaly. The first limnological records of the lake date from the turn of rhe century when the ltalian geographer BATIISTI (1898) gave a general physical description of i t. ToMASI (1963) in his general census of Trentino lakes did nor menrion any particular problems and described Lake Serraia as having a good biological equilibrium of its waters. From the data presenred, the lake appeared to be meso-eutrophic with a typical cyprinid fish population. In rhe mid-1970s, rhe lake was described as eutrophic (VITIORI 1977). Deterioration berween rhe mid-1960s and the mid-1970s was attributable to raw sewage inputs from a catchment area which saw a large increase in summer tourist populations an d a general increase in the standard o f living. Sewage inflow was diverred from the lake in 1986-87. From rhe mid-1990s, agriculrural practises in the carchment have changed from predominantly extensive (grassland) to intensive agriculture with aboveground (tunnel) and soi! cultivation of strawberries and other small fruit. Presenrly, there are 2.6 ha of tunnels and 3.9 ha of soil cultivation of strawberries and other small fruit crops in the carchment basin (personal communication, Agricultural Department). Tunnel cultivation presents particular problems with nutrient runoff. Each hectare of runnel produces a runoff of 1500 m of solution conraining rhe following quantities (kg) of nutrients: 379 Ca, 360 N, 249 K, 121 Mg, 87 P and 12 S (LEITEN 1995). A few of the larger tunnels (>l ha) are locared on the lakes shore and runoff presently goes directly into Lake Serraia. The lake is used for recreational purposes so its ecological state has important economic consequences. Since 1995, the lake has developed serious algal blooms, which in August 1997 caused the closure of the lake for recreational use with serious consequences for the local economy. We examine the results of long-term data on rhe lake and how they assist wirh understanding of increased enrichment.