We investigated the development of a heterotrophic plankton food web with or without phytoplankton primary production in a long-term (>1 yr) laboratory experiment. Water from 3 Swedish lakes (humic, oligotrophic clearwater, eutrophic) was exposed to low light or kept in total darkness in triplicate 100 l cylinders. Dissolved organic carbon (DOC) dynamics, bacterial growth and biomass of protozoans, rotifers and microcrustaceans were followed over 18 mo. In the dark treatments, no primary production was detected and DOC concentrations decreased by between 19 and 27% (1.3 to 3.2 mg C l -1). There was bacterial and protozoan growth in the dark during the whole experimental period. However, numbers and production of bacteria, as well as protozoan biomass, were significantly lower in darkness. Dissolved (DOM) and particulate organic matter (POM) initially present in the water (i.e. 18 mo old at the end of the experiment) helped to support substantial metazoan biomasses in dark treatments in the humic and eutrophic waters, but not in the oligotrophic clearwater lake. DOM in the humic water, thus largely of allochthonous origin, gave the highest and most prolonged support to zooplankton living in darkness. Our study indicates that a microbial food web, based on allochthonous organic matter and developing independently from phytoplankton, can act as a link to metazoan zooplankton, especially in oligotrophic humic lakes. These results confirm studies using stable C isotopes, showing a substantial incorporation of terrestrial carbon into zooplankton.KEY WORDS: Dissolved organic matter · DOM · Bacteria · Microbial food web · Zooplankton · Lake · Terrestrial carbon Resale or republication not permitted without written consent of the publisher

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1. Recreational and subsistence angling in South African estuaries is a small, but relevant, sector of South Africa's diverse coastal fisheries. This paper reviews these fisheries and their effects in two estuaries.  2. The composition of angler's catches in two large Eastern Cape Province estuaries has hardly changed over a period of nearly 20 years.  3. Gill‐net catches (in terms of CPUE) indicate a decline in the abundance of the most popular angling fish species, especially the spotted grunter Pomadasys commersonnii.  4. Regular freshwater inputs into estuaries are required to ensure a salinity gradient which is crucial for the maintenance of an abundant and diverse estuarine ichthyofaunal assemblage.  5. Present management and conservation policies are poorly defined, and the philosophy and principles of ecosystem preservation, rather than that of species preservation, should be seriously considered. © 1996 by John Wiley & Sons, Ltd.

We examined the response of bacteria and proto-and metazooplankton to photomodified dissolved organic matter (DOM). Sterile filtered water from a eutrophic and a humic lake, that was either exposed to artificial ultraviolet (UV) radiation or kept dark, was added to semicontinuous laboratory microcosms that lasted for 7 weeks. Bacterial production responded positively to photochemical modification of DOM regardless of lake type. Final heterotrophic biomass (bacteriaϩprotoϩmetazooplankton) was 47 Ϯ 5 and 37 Ϯ 5 g carbon (C) L Ϫ1 in microcosms with UVexposed and unaltered eutrophic water DOM and 15 Ϯ 4 and 11 Ϯ 2 g C L Ϫ1 in microcosms with UV-exposed and unaltered humic water DOM, respectively. For the eutrophic water, there were no significant differences in proto-or metazooplankton biomasses between microcosms receiving UV-exposed or nonexposed DOM. Differences between eutrophic water microcosms were not significant when flagellates, ciliates, cladocerans, and copepods were examined separately. In microcosms with UV-exposed humic water, biomasses of heterotrophic flagellates, rotifers, nauplii, and cladocerans were higher than in those with nonexposed DOM. Higher final metazooplankton biomass following addition of UV-exposed humic water indicates that photochemically modified DOM can be effectively transferred through the microbial loop.

The mixotroph Poterioochromonas malhamensis was grown in batch and semicontinuous cultures in order to examine the dependence of phagotrophy versus phototrophy at bacterial densities similar to those found in oligo-and mesotrophic lakes in combination with differing light regimes. In addition, the growth rates and biomass accumulation of the mixotroph were compared to those of the heterotroph Spumella elongata grown under similar conditions. In P. malhamensis, primary production rates and cell-specific chlorophyll a concentrations were significantly higher when grown at low bacterial densities, and the carbon cell -1 gained by photosynthesis corresponded to 79% of the carbon cell -1 gained by bacterial ingestion. Furthermore, due to a longer exponential growth phase, cell densities and biomass of P. malhamensis were significantly higher when the mixotroph was cultured in high light than in darkness. As a result, the cumulative biomass of the mixotroph was significantly higher in the light treatments than the biomass of the heterotroph at similar bacterial densities. These results suggest that photosynthesis may contribute more to the nutrition of P. malhamensis in lakes with low bacterial abundances than previously suggested, and that the mixotroph's capacity for photosynthesis enables it to dominate in terms of biomass over the heterotroph. KEY WORDS: Poterioochromonas malhamensis · Spumella elongata · Phagotrophy · Photosynthesis · Competition Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 36: [247][248][249][250][251][252][253][254][255][256] 2004 mixotrophic nutrition (Raven 1995(Raven , 1997. Thus, in the competitive relationship between mixotrophs and obligate heterotrophs, the former are believed to have an advantage only when prey densities are low and light is available (Jones 2000).The aims of this study were (1) to examine and compare growth and biomass accumulation of the mixotrophic flagellate Poterioochromonas malhamensis and the heterotrophic flagellate Spumella elongata grown under similar conditions in batch and semi-continuous cultures and (2) to further investigate the effect of light and bacterial densities similar to those found in oligotrophic and mesotrophic lakes on the reliance of phagotrophy versus phototrophy in P. malhamensis. Small nanoflagellates belonging to the genus Spumella spp. are very abundant in many lakes (Auer & Arndt 2001), and have been reported to be important consumers of bacteria (Weisse 2002). The mixotrophs Poterioochromonas spp. and closely related Ochromonas spp. are abundant bacterivores that, as a result of their nutritional flexibility, have received considerable interest over the years (e.g. Myers & Graham 1956, Handa et al. 1981, Lewitus & Caron 1991, Sanders et al. 2001. P. malhamensis is an efficient bacterivore, and it has been suggested that the flagellate relies on phagotrophy to obtain most or all of the energy used for metabolism and growth ). Furthermore, when actively growing on bacteria, ...