Planktonic Food Web Structure along the Sau Reservoir (Spain) in Summer 1997

We studied the planktonic food web in eutrophic Sau Reservoir (Catalonia, NE Spain). Along the longitudinal axis from the Ter River downstream to the dam, we characterized a microbial succession of food web dominance of bacteria‐HNF‐ciliates. The Ter River transports a large load of organic material into the reservoir, with a bacterial density of ∼9 · 10⁶ large cells per ml. While at the first lacustrine station of the Reservoir HNF were the dominant bacterial consumers, at the others, an oligotrich ciliate, Halteria grandinella, was the main protozoan bacterivore. Most of the bacterial production in the reservoir epilimnion was consumed by grazing. The spatial succession of the reservoir microbial food webs was followed downstream by maximum densities of their potential predators among zoo‐plankters – rotifers, and early developmental stages of copepods.     

Spectacular abundance of ciliates in anoxic pond water: contribution of symbiont photosynthesis to host respiratory oxygen requirements

Abstract: Very large numbers (3466 ml⁻¹) of ciliated protozoa were found living beneath the oxic-anoxic boundary in a stratified freshwater pond. Most ciliates (96%) contained symbiotic algae ( Chlorella spp.). Peak abundance was in anoxic water with almost 1 mol free CO₂ m⁻³ and a midday irradiance of 6 μmol photon m⁻² s⁻¹. Photosynthetic rate measurements of metalimnetic water indicated a light compensation point of 1.7 μmol photon m⁻² s⁻¹ which represents 0.6% of sub-surface light. We calculate that photosynthetic evolution of O₂ by symbionts is sufficient to meet the demand of the host ciliates for 13 to 14 hours each day. Each 'photosynthetic ciliate' may therefore become an aerobic island surrounded by anoxic water.     

A comparative fine structural and phylogenetic analysis of resting cysts in oligotrich and hypotrich Spirotrichea (Ciliophora)

So far, neither morphology nor gene sequences have provided a reliable classification of halteriid and hypotrichid spirotrichs. Thus, we performed a comparative study on the fine structure of the resting cysts in some representative species, viz., the oligotrichs Halteria grandinella and Pelagostrombidium fallax and the oxytrichid hypotrichs Laurentiella strenua, Steinia sphagnicola, and Oxytricha granulifera. Main results include: (i) there are three different, very likely non-homologous cyst surface ornamentations, viz., spines (generated by the ectocyst), thorns (generated by the mesocyst), and lepidosomes (produced in the cytoplasm); (ii) Halteria has a perilemma; (iii) Halteria, Meseres and Pelagostrombidium have fibrous lepidosomes, while those of Oxytricha are tubular; (iv) the cyst wall structure of Pelagostrombidium and Strombidium is distinctly different from that of halteriids and oxytrichids, which are rather similar in this respect; (v) cyst ornamentation does not provide a reliable phylogenetic signal in oxytrichid hypotrichs because ectocyst spines occur in both flexible and rigid genera. The new observations and literature data were used to investigate the phylogeny of the core Spirotrichea. The Hennigian argumentation scheme and computer algorithms showed that the spirotrichs are bound together by the macronuclear reorganization band, the subepiplasmic microtubule basket, and the apokinetal stomatogenesis. The Hypotrichida and Oligotrichida are united by a very strong synapomorphy, viz., the perilemma, not found in any other member of the phylum. Halteriid and oligotrichid spirotrichs form a sister group supported by as many as 13 apomorphies. Thus, the molecular data, which classify the halteriids within the core hypotrichs, need to be reconsidered.     

Salinity and fish effects on Salton Sea microecosystems: zooplankton and nekton

The Salton Sea is the largest inland lake in California. Currently (1997) the salinity of the lake is about 44 g l-1 and is increasing gradually as a result of continued agricultural wastewater inflows, high evaporation rates, and lack of an outlet. A microcosm experiment was carried out to determine the effects of salinity (30, 39, 48, 57, and 65 g l-1) on Salton Sea algae and invertebrates in outdoor aquatic microcosms. The experiment was also designed to assess the effects of tilapia ( Oreochromis mossambicus) on this community at two of these salinities (39 and 57 g l-1). Fiberglass tanks containing Salton Sea water were adjusted to the appropriate salinity by the addition of salts, identically inoculated with organisms from the Salton Sea and other saline water bodies in the region, and monitored for 15 months. Planktonic and nektonic invertebrates were sampled monthly at night from the upper part of the water column. The dominant invertebrates present were Gammarus mucronatus, Artemia franciscana, Trichocorixa reticulata, and an assemblage of ciliate protozoans. Gammarus decreased and Trichocorixa increased with increasing salinity. Artemia was present only at the two highest salinities. Rotifers, harpacticoid and cyclopoid copepods, barnacle larvae, and protozoans all showed marked and varied responses. During the latter half of the experiment, the invertebrate assemblage was dominated by Gammarus at 30 and 39 g l-1, by protozoans at 48 g l-1, and by protozoans and Trichocorixa at 57 and 65 g l-1. The presence of tilapia caused a 99 percent reduction in Gammarus at 39 g l-1 and a 70–90 percent decrease in Trichocorixa at 57 g l-1. These were accompanied by substantial increases in rotifers, copepods, and certain protozoans, and decreases in other protozoans. As the salinity of the Salton Sea continues to increase, large changes in the invertebrate populations are expected. This study suggests that the principal change would be an increase in Trichocorixa densities, the loss of Gammarus, and the appearance of Artemia at about 60–70 g l-1, when both fish and invertebrate predators are likely to be scarce or absent. Protozooplankton abundance is likely to increase when tilapia declines and later decrease when and if large Artemia populations develop. This revised version was published online in July 2006 with corrections to the Cover Date.