Small-scale nutrient patchiness: Some consequences and a new encounter mechanism’

Semicontinuous cultures of Lake Michigan algal assemblages were run at O.l5*day-’ dilution rates under P limitation for 41 days. In control cultures, nutrients were supplied to the entire culture once per day. In patch cultures, 10% of the cultures received a high concentration IO-min pulse each day. Patch and control cultures received equal nutrient flux and achieved equal biomass levels; however, their species composition was different. Patch cultures were dominated by the filamentous blue-green alga Schizothrix calcicola and control cultures were dominated equally by S. calcicolu, Nitzschia aciculuris, and a group of phytoflagellates. These results, as well as shifts in subdominants, demonstrate the effect of small-scale, short term nutrient patches on species composition. Similar patch and control unialgal cultures revealed lower cell nutrient quotas for patchy cultures. Microvideographic observations of Daphnia magna suspended in three concentrations of Chlamydomonas sp. allowed quantification of rates at which algal cells were collected, swept close by, and released via bolus rejection or outwashing of unpacked cells. Outwashing release rates of presumably unharmed cells increased from 3 1 to 7 1% of the collection rate as food concentration increased. It appears that algal outwashing is a significant mechanism for eliminating excess food. An important implication is the potential algal nutritional consequence of this very close nondestructive encounter. In fresh and marine waters when nutrients are scarce and where zooplankton supply most of the N and P, supply is likely to be heterogeneous (Lehman and Scavia 1982a,b). If competition for those scarce resources occurs within patches of relatively high concentration rather than in uniformly low concentrations, then the “rules” of resource-based competition may be different than described previously (e.g. Tilman 1980). While the existence of such patches and their potential relevance to ecosystem dynamics can be debated on theoretical grounds, empirical information must be brought to bear on the issue. Small-scale nutrient patchiness and its effects on community structure are difficult to measure in nature; however, careful laboratory experiments can provide insight into potential ’ GLERL Contribution 388. consequences. Nutrient temporal patchiness, explored through a series of comparisons between “pulse-and-chase” and continuous-nutrient-supply continuous cultures (e.g. Turpin and Harrison 1979, 1980), resulted in phytoplankton assemblages that differ from those given temporally uniform supplies. To explore the ecological significance of zooplankton-produced or other spatially discrete, nutrient patchiness, an experiment should provide spatial and temporal nutrient heterogeneity. This is because only a portion of a community will experience elevated nutrient concentrations. We maintained natural algal assemblages in semicontinuous cultures with nutrientaugmented, filtered lake water supplied either uniformly or in discrete patches to explore the influence of small-scale nutrient patchiness on Lake Michigan species composition. We also ran patch and control semicontinuous unialgal cultures at equal