Small‐scale nutrient patchiness: Some consequences and a new encounter mechanism1

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Abstract
Semicontinuous cultures of Lake Michigan algal assemblages were run at 0.15·day‒1 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 10‐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. calcicola, Nitzschia acicularis, 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 31 to 71% 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.