Patrizia Comoli

The decline of Daphnia hyalina galeata in Lago Maggiore: a comparison of the population dynamics before and after oligotrophication

Abstract: We reconstruct the impact of different environmental cues on Daphnia seasonal dynamics by means of an analysis of demographic and reproductive parameters, as well as of the size structure of the population. Data from 1996, indicative of the most recent productivity of the lake, with those from 1973, when the lake was meso-eutrophic, allow a discussion of the relevance of the observed changes for the structure of the pelagic food web of Lago Maggiore. Daphnia summer dynamics is mainly controlled by Bythotrephes longimanus. This is quite a different situation from that of the past, when Leptodora kindtii was the prevalent predatory cladocera. The size structure of Daphnia population revealed a depletion of small ovigerous females with increases in the predator. Our data allow a discussion of the idea, recently proposed, of Bythotrephes longimanus as a visual sit- and -wait predator.

Reconstructing long-term changes in Daphnia's body size from subfossil remains in sediments of a small lake in the Himalayas

A combined analysis of modern zooplankton and fossil Cladocera assemblages from a Himalayan lake, Lake 40, revealed that the endemic Daphnia tibetana disappeared in the late-1980s, after persisting as the only Daphnia species for almost 3000 years. Daphnia head shields, which are rarely recovered from the sediments, were the most abundant Daphnia remains in the lake. The remains were of the Ctenodaphnia type; the smaller ones had a large central hole. Head shields of the same type were also found in zooplankton water samples, rich in exuviae as well as of intact specimens of D. tibetana, from a nearby lake. Small individuals had a distinct nuchal organ in the dorsal region of the head. We therefore postulated that the head shields with the hole were from young (newborn or individuals in the two first moults). Up to now, the nuchal organ has been described in laboratory populations of D. (Ctenodaphnia) magna, where it disappears quite early in life. It probably functions as an osmoregulatory organ, essential for the survivorship of late embryos and early juveniles. On the other hand, as far as we know, head shields with a hole have never been recorded in plankton or in sediments. In view of the fact that head shields were representative of Daphnia abundance, we used them to reconstruct changes in density and body size during ca. 3000 years. In fishless mountain lakes, mean Daphnia body size tends to increase toward the end of the productive season. The number of moults and maximum body size depend mainly on the duration of the ice-free period, and on summer temperatures. In cold years, when the productive season is short, the number of moults will be low, and the range of Daphnia body size will be narrow. We used Daphnia body size and abundance estimates, in addition to an analysis of changes in the Cladocera assemblage, to reconstruct past environmental conditions.

Evidence for short-lived oscillations in the biological records from the sediments of Lago Albano (Central Italy) spanning the period ca. 28 to 17 k yr BP

We report the results of analyses of pigments (derived from algae and photosynthetic bacteria), diatoms and invertebrate fossil remains (ostracods, cladocerans, chironomids) in two late Pleistocene sediment cores from Lago Albano, a crater lake in Central Italy. The record contains evidence for oscillations in lake biota throughout the period ca. 28 to 17 k yr BP. The earliest of these are contained in the basal 3.5 m of light olive-gray and yellowish-gray spotted muds sampled in core PALB 94-1E from 70 m water depth. The later oscillations are best represented in the more extended sediment sequence recovered from a second core site, PALB 94-6B, in 30 m water depth. The sediments at site 1E, containing the earlier oscillations (ca. 28-24 k yr BP), predate any sedimentation at the shallower site, from which we infer an initially low lake level rising to permit sediment accumulation at site 6B from ca. 24 k yr onwards. At site 6B, massive silts rich in moss remains are interbedded with laminated silts and carbonates. These sediments span the period ca. 24 to 17 k yr and are interpreted as representing, respectively, times of shallow water alternating with higher lake stands, when the lake was stratified and bottom water was stagnant. A range of mutually independent chronological constraints on the frequency and duration of the oscillations recorded in the lake biota indicate that they were aperiodic and occurred on millennial to century timescales. We interpret them as responses to climate forcing through its impact on lake levels and changing aquatic productivity. The time span they occupy, their frequency and their duration suggest that at least some of these changes may parallel both the Dansgaard-Oeschger events recorded in Greenland Ice Cores and the contemporary oscillations in North Atlantic circulation documented in marine sediment cores.

A late glacial and holocene record of biological and environmental changes from the crater Lake Albano, Central Italy: An interdisciplinary european project (PALICLAS)

This paper reports the results of biological analyses (pigments, diatoms, chrysophyte cysts, cladocerans, chironomids and ostracods) of a ca. 14 m-long sediment core recovered from Lake Albano (Central Italy) in the course of the EU-funded project PALICLAS (PALaeoenvironmental analysis of Italian Crater Lake and Adriatic Sediments).A reconstruction of the environmental evolution and ecosystem response of Lake Albano during the last ca. 30 kyr was possible. Additional information on lake level oscillation is obtained from benthic and planktonic palaeocommunities. Several oscillations in the productivity and the level of the lake were detected in the oldest sediment layers (from ca. 30 kyr BP to ca. 17 kyr BP), followed by a long (ca. 5 kyr BP) period of low productivity in which cold, holomictic conditions prevailed. A period of high biological activity and, probably, meromictic conditions during the early-mid Holocene was detected. A clear impact of human activities in the catchment was found at ca. 4 kyr BP in the form of increased erosion, associated with a decline in the abundance of biological remains. Further signs of human impact on the lake ecosystem are recorded during the Roman period. Although large-scale environmental changes (e.g. regional climate changes) caused many of the observed biological changes, human activities were important during the mid-late Holocene.

An unusual type of Daphnia head shields from plankton and sediments of Himalayan lakes.

The paper reports the finding of a particular type of Daphnia (Ctenodaphnia, Dybowski & Grochowski 1895) head shields, found in the plankton and in the sediments of two lakes in the Khumbu Region (Nepalese Himalayas). The capsules, all of the smallest size, were characterised by the presence of a hole in the medio-dorsal region, behind the posterior median groove. The location of the hole suggests a link with the nuchal organ, which we found in the neonates of the species. The finding is important for taxonomy and provides information for discussing the current hypothesis on the biogeography of the Ctenodaphnia subgenus. Being well preserved in sediment cores, the capsules are useful tools for reconstructing past phases of active reproduction which occurred also in the past.

Length-specific carbon content of theDaphnia population in a large subalpine lake, Lago Maggiore (Northern Italy): The importance of seasonality

We compared three methods for calculating the standing stock biomass ofDaphnia hyalina galeata population in Lago Maggiore during a single year. We applied body length/carbon regression equations (LCR) obtained under controlled laboratory conditions and directly from field data on carbon and body length to field data on length-specific abundance. Laboratory-derived equations satisfactorily described our field estimates of length-specific carbon content in spring. As the length-standardized carbon content decreased exponentially along the season, we included time of the year as an auxiliary variable, and tested whether it affected slope, intercept or both. The decrease had no effect on slope but reduced the intercept of the LCR. We also calculated biomass, using a single mean value from the literature. By comparing the results with those obtained by simple conversion of individuals number into carbon we might discriminate between the effect of changes in the size frequency distribution of the population and the decrease with time of the length-standardized carbon content. Although we tested the different models for non-ovigerous females, the results did not change when we included also data on females with eggs in the first three developmental stages, whereas they changed substantially when we included females with embryos. We formulate two hypotheses to explain this result and discuss possible implications for the calculation of the standing stock biomass ofDaphnia population.