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Submerged macrophytes play crucial roles in maintaining the stability of clear-water states in shallow lakes. Recent stable isotope studies have shown that crustacean zooplankton can utilize submerged macrophyte carbon, but macrophytes alone cannot support the growth and reproduction of such grazers, being deficient in highly unsaturated fatty acids (HUFA). We hypothesized that flagellates feeding on macrophytes can synthesize HUFA and thereby support crustacean zooplankton. To test this hypothesis, we conducted a feeding experiment in which Daphnia magna were provided with a diet of submerged macrophyte Hydrilla verticillata detritus which had been degraded by lake microbes. The chlorophyte Scenedesmus bijuga and undegraded macrophyte detritus were used as controls for comparison of Daphnia's performance. Using biochemical analysis, we examined how the degradation process affected the food quality of the macrophyte. Flagellates were subsequently isolated from the degraded macrophyte and cultured heterotrophically to detect their HUFA synthesis. The 5-day degraded H. verticillata showed significantly higher HUFA concentrations than undegraded macrophyte detritus. They supported better Daphnia performance than undegraded macrophyte, being comparable with S. bijuga. Two isolated flagellates (SL-1 and SL-2), identified as Ochromonas sp. and Poterioochromonas sp., were found to contain HUFA when cultured heterotrophically without dietary sources of fatty acids, suggesting their HUFA synthesis ability. Our results demonstrate that submerged macrophytes may thus indirectly support crustacean zooplankton via flagellate mediation. As crustacean zooplanktons are of key importance for water quality in the grazer control of phytoplankton, this microbial facilitation may contribute to the maintenance of macrophyte clear-water conditions in shallow lakes. [ABSTRACT FROM AUTHOR]

In temperate shallow lakes, submerged macrophytes facilitate zooplankton development by providing refuge against fish predation and, thereby, indirectly contribute to maintaining a clear-water state through enhanced zooplankton grazing. The role of macrophytes for zooplankton and their grazing potential is less clear for tropical lakes. We investigated crustacean zooplankton in a phytoplankton-dominated basin (algal basin) and two restored basins dominated by macrophytes (macrophyte basins) in the shallow Huizhou West Lake in tropical southern China. We found that copepods prevailed in all basins, but the dominant taxon differed, with omnivorous cyclopoids dominating in the algal basin and herbivorous calanoids in the macrophyte basins. Moreover, the biomass ratios of calanoid:copepod and zooplankton:phytoplankton were higher in the macrophyte basins than in the algal basin. Our results suggest that restoration measures involving macrophyte transplantation and fish removal lead to reduced fish predation on zooplankton, which help to maintain the clear-water state when macrophytes are established due to higher control on phytoplankton. However, unlike in temperate lakes, large-bodied Daphnia were generally absent and the zooplankton:phytoplankton ratio was overall low, indicating a weaker top-down control in tropical lakes, which is likely due to higher fish predation. [ABSTRACT FROM AUTHOR]

Submerged macrophytes cannot be utilized directly by zooplankton. However, vegetation can serve as an organic carbon resource for heterotrophic bacteria, which are themselves accessible to zooplankton. We therefore hypothesize that submerged macrophytes supply a carbon source to zooplankton by increasing the availability of food such as heterotrophic bacteria. Here, we used stable carbon isotope (13C) labeling to trace the carbon flow from submerged macrophytes to zooplankton with a mesocosm experiment. The carbon stable isotope ratios of zooplankton and their potential food sources were analyzed in mesocosms planted with 13C-labeled Hydrilla verticillata (L. f.) Royle in comparison with the control in which plastic plants were planted. We found that all potential food resources of zooplankton, including phytoplankton, bacterioplankton, macrophyte-associated epiphyton and epibacteria, were significantly enriched with 13C in the presence of 13C-enriched H. verticillata. Zooplankton were significantly more enriched with 13C than phytoplankton, epiphyton and bacterioplankton but significantly less enriched with 13C than epibacteria. Based on a stable isotopic mixing model, we found a macrophyte carbon contribution of 30.5% to epibacteria and 14.7% to zooplankton. Our results indicated that macrophytes might be used by zooplankton as a carbon resource, mainly via a pathway involving epibacteria attached to macrophytes. [ABSTRACT FROM AUTHOR]

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