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.     

Superior life history traits of Micromoina arboricola in a humic over a non-humic environment

Micromoina arboricola, one of the smallest moinids on record, is currently known from a single tree hole in a tropical forest, containing a small volume of humic water, in quasi absolute darkness. We test whether it has been refuted to this environment by superior competitors, or whether it is genuinely adapted to this life space by culturing it in its native environment, and in a clear water environment supplemented with algal food, yeast and fermented fish ration. Only maturation time is similar between both treatments. Total offspring, number of clutches, average clutch size and longevity are higher in the humic treatment. Population growth is initially higher in the clearwater treatment, but then collapses. In the humic treatment there is a steady ascent towards a stable plateau. We conclude that the species is better adapted to its microcosm than to a ‘normal’ aquatic biotope.     

2000～2002年柘林湾浮游枝角类的生态特征

2000年7月～2002年7月的调查结果表明,广东柘林湾浮游枝角类有3种,隶属于2科3属.其中,鸟喙尖头氵蚤(Penilia avirostris)为全年优势种,合计占浮游动物总个体数的9.08%和枝角类总个体数的91.84%.该湾浮游枝角类的密集区位于黄冈河、河口区域及湾外水域,其数量与盐度呈显著负相关关系.浮游枝角类的季节变化极为显著,表现为夏、秋多,冬、春少的基本格局,其数量与水温呈显著正相关关系.     

Environmentally-induced plasticity of life history parameters in the cladoceran Simocephalus vetulus

Key life history parameters significantly differed between two different clones of Simocephalus vetulus originating from different habitats, i.e. a fishless pool and a fish pond. The following fitness-related traits were studied: lifespan, age at first reproduction, interclutch period, number of clutches, and number of embryos. In general, in relation to the fish pond clone the fishless pool clone revealed longer lifespan and higher fecundity due to more frequent clutches, contributing to a higher fitness. Higher food quantities decreased the lifespan but did not alter fitness-related traits except for a shorter interclutch period. The Simocephalus clone from the fishless pool matured earlier in low food conditions, while the fish pond clone matured earlier in the high food conditions. The switch in food concentration between the subsequent generations significantly shortened the lifespan in the fish pond clone but revealed no effect in the fishless pool clone.     

Application of sediment traps in global change research in mountain lakes

Representativity of biota in the biosphere also have similar significance in the lithosphere after their death. Recently, taphonomic representation of biota in the lithosphere has been widely used for minimizing the biases in palaeo-environmental reconstructions including global climate change. In September 2001, a sediment trap was installed at the central area of the Loch Coire Fionnaraich （LCFR）, a small, oligotrophic mountain loch in northwest Scotland for a taphonomic study of cladoceran zooplankton. The trap was emptied on a seasonal basis, where an average 0.14 g dry material was recovered in each season, January, May and August 2002 respectively. A sediment core was also taken from near the sediment trap location. The top 0.5 cm section, the ＇core＇ was used for a comparative study with the trap sediment samples. The planktonic Cladocera consisted of Bosmina coregoni and appeared to have excellent preservation in the trap assemblages. Moreover, some common littoral Cladocera such as Alonella excisa and Aloha guttata appear to be less well preserved in the fossil assemblages. Large littoral taxa such as Eurycercus lamellatus and Aloha quadrangularis were rare in the trap but relatively common and well preserved in the sediment-water interface. It is suggested that taphonomic processes play a dominant role on deposition of fossil remains of Cladocera and caution should be taken while inferring the past environmental changes whilst using fossil assemblages recovered from the lake centre as a representation of the entire cladoceran community in the lake.     

Ratio of neonate to adult size explains life history characteristics in cladoceran zooplankton

The size ratio of adults to neonates varies greatly in cladocerans across taxa. In this study we tested the impact of this ratio (using length basis) on the selected life history parameters of nine species of freshwater cladocerans (Alona rectangula, Pleuroxus aduncus, Scapholeberis mucronata, Moina macrocopa, Moina wierzejskii, Ceriodaphnia dubia, Daphnia laevis, Diaphanosoma birgei and Daphnia pulex). Experiments were conducted on a mixed diet of Chlorella vulgaris (1.0·10⁶ cells mL^–1) and Scenedesmus acutus (0.5·10⁶ cells mL^–1) at (22 ± 1)°C. Using a cohort life table experimental design, we quantified age‐specific reproductive output. Body sizes of neonates and adults as well as dry weight were determined using standard procedures. We found that the ratio of neonate to adult body length decreased with increasing body size of the adult. In terms of clutch size, however, smaller taxa produced lower numbers of offspring than larger ones. The influence of ratio neonate/adult size on average lifespan, age at first reproduction and at the maximum clutch showed U‐shaped curves. In smaller taxa (< 700 μm) and larger taxa (> 1500 μm) the ratio between the neonate and adult body size was less than 0.4 or more than 0.6. In medium sized taxa (1000?1500 μm), the ratio of neonate to adult size was between 0.4 and 0.6. The results have been discussed in relation to the life history strategies of cladocerans.     

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.     

Sensitivity of tropical-African aquatic invertebrates to short-term trends in lake level and salinity: a paleolimnological test at Lake Oloidien,Kenya

Aquatic invertebrates are intrinsically capable of rapid and sensitive response to changes in their lacustrine habitat. Fossil invertebrate assemblages preserved in the sediments of a climate-sensitive lake can thus produce high-resolution proxy records of past climate. In shallow lakes, however, a potential conflict exists between the sensitivity of biota to frequent habitat change in their fluctuating environment and the increased probability of disturbance of selected proxy records by bioturbation and physical mixing of sediments. I investigated this problem with tropical-African aquatic invertebrate faunas in a paleolimnological sensitivity study that incorporates both the response of biota to short-term habitat change and the taphonomic integrity of fossil assemblages in a small, shallow, and hydrologically closed lacustrine basin. Analysis of chironomid, cladoceran, and ostracode remains in a²¹⁰Pb-dated short core from Lake Oloidien (Kenya) indicates that habitat changes accompanying the late 19th- and 20th-century fluctuations in lake level (Z _max range: 3 to 18 m) and salinity (conductivity range:c. 400 toc. 2000–4000 µS cm^–1) were sufficient to drastically alter the composition of local benthic and planktonic invertebrate faunas. This response remained relatively unaffected by taphonomic phenomena during its incorporation into the sediment record. Results indicate that tropical-African aquatic invertebrate faunas in suitable climate-sensitive lakes are a valuable tool to resolve paleoclimatic fluctuations on a timescale of decades.This paper was presented at the VI Palaeolimnology Symposium, held at Canberra in April 1993. Dr. Mark Brenner guest edited this contribution.