Heike Zimmermann-Timm

Seasonal Dynamics of Aggregates and their Typical Biocoenosis in the Elbe Estuary

In 1995, an extensive investigation was carried out in the Elbe Estuary in Germany. Special attention was paid to organisms, including bacteria, amoebae, ciliates, flagellates, rotifers, larvae ofDreissena polymorpha, and nematodes, dispersed in the water column and associated firmly or loosely with different types of aggregates. The abundance, size, and colonization by microorganisms of the aggregates varied during the seasons. The largest aggregates were found during spring and summer, when diatoms, rotifers, and crustaceans were present. The most aggregates were encountered in spring and from summer to autumn. Most of the species observed during the study were more common in pelagic habitats than in benthic ones. However, the presence of ciliates in the groups Hypotrichida and Sessilida as well as as flagellates in the groups Biosoecida, Cercomonadida, Choanoflagellida, Chrysomonadida, Kinetoplastida, and amoebae and some nematodes in the open water depends, upon the availability of surfaces, because they are sessile or poor swimnters, and some flagellates and amoebae need to attach themselves to an object to feed. Most organisms were much more abundant in or on aggregates than in the surrounding water during spring and summer, which indicates that aggregates enhance the habitat and promote the development of the organisms. From spring through autumn, the structure of the community associated with the aggregates is influenced by the pelagic environment. The presence of the species in the benthic community was detected only during summer.

Longitudinal and Transverse Distribution of Plankton Rotifers in the Potamal of the River Elbe (Germany) during Late Summer

In July and August 1999, the longitudinal and transverse spatial dynamics of plankton rotifers and phytoplankton, the chlorophyll a content and abiotic parameters, were investigated in the potamal of the River Elbe, Germany during low river discharge. Longitudinal changes in composition and abundance were studied at 9 sampling stations over a distance of 537 river kilometers. In total 42 rotifer taxa were identified. Species composition and abundance were remarkably distinct from those of other large lowland rivers. Rotifers exhibited a steady downstream increase in abundance, with a maximum up to 12135 individuals l–1 at the most downstream station with Trichocerca pusilla dominating (37.0–77.7% of total abundance) the community at all stations. Phytoplankton abundances peaked in the middle part of the potamal (km 317) with 224 · 106 cells · l–1. At all stations small centric diatoms (<25 μm) and green algae (coccales) dominated the phytoplankton community. Small-scale transverse variations of abundance were investigated by sampling 6 transects of the main stream and the corresponding groyne fields in the central section (km 422) of the stretch of the river that was examined. Statistical analysis revealed no significant differences in abundance of rotifers and phytoplankton between main stream and groyne fields.

Aggregate-associated bacteria and heterotrophic flagellates in the River Elbe : Their relative significance along the longitudinal profile from km 46 to km 583

The aim of this study was to investigate the occurrence of microaggregates and their colonization by bacteria and heterotrophic flagellates along the longitudinal profile of the River Elbe from km 46 in Dresden up to km 583 near Hamburg. As additional parameters, the content of chlorophyll a, the dry weight of the suspended particles and the concentration of nutrients in the river were measured. Abundances of aggregates ranged between 322 and 1196 aggregates ml -1 with the lowest value at the uppermost location. Aggregate dry weight also showed the lowest value at this location with a mean of 0.02 g l -1 . The chlorophyll a-content revealed a gradual increase from the uppermost to the lowest reach with a maximum of 216 μg l -1 at km 583. The total abundances of bacteria and heterotrophic flagellates reached values of 9.9 . 10 6 cells ml -1 and 1.2 . 10 3 cells ml -1 , respectively. The percentages of aggregate-associated cells in the river water ranged from 2.2 to 13.3% in the case of bacteria and from 16.7 to 73% for heterotrophic flagellates. The highest values occurred in both cases in the midreaches between km 475 and km 275; these coincided with high concentrations of aggregates. In general, low abundances of cells were detected in the upper part of the river, but high values were reached when numbers of cells were related to aggregate volume and dry weight.

Feeding behaviour of two planktonic freshwater ciliates coexisting during spring time in the eutrophic Belauer See (Bornhöveder Seenkette, North Germany)

In spring 1991, we investigated the development of the planktonic community of a eutrophic lake (Belauer See, North Germany), focusing on the feeding behaviour of Vorticella rhabdostyloides and Tintinnidium fluviatile, the two most abundant ciliate species in the pelagic zone of this lake during spring. Experiments on the uptake of fluorescently labelled particles in different size classes and fluorescently labelled bacteria and algae permitted the determination of the importance of those kinds of food. Vorticella rhabdostyloides ingested particles about 1.0 lam and 3 gm long and autotrophic picoplankton (APP). Tintinnidium fluviatile preferred particles 0.5 ~m to 1.0 ~tm and 5.0 gm long, APP and diatoms in the genus Nitzschia. Ingestion rates ranged between 100 and 120 particles h 1 for each TintimTidiumfluviatile and 60 to 230 particles h 1 for each Vorticella rhabdostyloides. APR bacteria, centric diatoms and nanoprotozoans were evaluated as foods for both ciliates. Tintinnidium fluviatile was found to exert a strong short-term control on picoplankton (rs = -0.941, n = 6, p < 0.05) and small centric diatoms (r s = 0.700, n = 6, p < 0.05). The different diets of both ciliates may explain the coexistence of both species during spring.

Beggiatoa leptomitiformis — a filamentous sulfur-oxidizing bacterium colonizing laboratory-made aggregates

Abstract The colonization of laboratory-made aggregates by the filamentous sulfur-oxidizing bacterium, Beggiatoa leptomitiformis , was investigated over a period of 14 days during winter and spring. The aggregates were generated from natural water from the freshwater zone of the Elbe Estuary using a tilting tube roller. Experiments were carried out under natural environmental conditions in a climate chamber. Within one hour after the beginning of the experiments, macroaggregates had already started to form. They reached a maximum size of 1 mm during the experiments. Both in winter and in spring, the water in which aggregates developed remained well saturated with oxygen. From the first day of the experiments, the sulfur-oxidizing bacterium, Beggiatoa leptomitiformis , was detected living in the aggregates and in the surrounding water. In the aggregates, the number of these bacteria was in the order of 10 6 ml −1 , whereas in the surrounding water, they were three orders of magnitude less abundant. In addition to the sulfur-oxidizing Beggiatoa , sulfate-reducing bacteria were detected within the aggregates.