Empirical evidence of chaos, or complex behavior, in ecosystems is scarce, presumably due to high system-level noise and/or the rarity of conditions necessary for complex behavior to arise. An alternative explanation might be that complex behavior is fragile and readily suppressed by disturbances that are common in many ecosystems. Here we investigated the role of disturbance frequency and magnitude on complex behavior and focused on population succession trajectories in a plankton system. Because of its prominence in aquatic ecology, we used hydraulic flushing and nutrient loading as disturbances. Our findings from numerical modeling exercises and laboratory microcosm experiments using natural plankton assemblages indicated that one aspect of complex behavior, divergence of nearby trajectories, was suppressed when the magnitude and periodicity of hydraulic flushing and nutrient loading were large. In other words, complex succession became determinable. Divergence of nearby trajectories was relatively robust, however, because pulses of not less than 85% of the total inflow were required to suppress this behavior. Our numerical findings also revealed that large hydraulic disturbances could introduce to the system another aspect of complex behavior, aperiodic succession.

Ecosystem form and functioning is affected by inflows, a relationship that has long been an interest of aquatic ecologists (Ketchum 1951, 1954, Brook & Woodward 1956). More recently, special sessions at Society of Environmental Toxicology and Chemistry (SETAC) and Association for the Sciences of Limno-logy and Oceanography (ASLO) conferences focused on this topic, with dedicated issues in Journal of Plankton Research (volume 33, 2011) and Canadian Journal of Fisheries and Aquatic Sciences (volume 69, 2012) following these meetings. The extent and composition of seagrass beds, diversity of periphyton communities and timing of algal blooms are examples of ecosystem forms that are influenced by in flows

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Productivity and community structure of phytoplankton and zooplankton are influenced by hydrologic disturbances in many ways. In a recent modeling study it was suggested that pulsed inflows might enhance zooplankton performance, curb accumulation of phytoplankton accumulated biomass, and promote phytoplankton species diversity. We tested these predictions by performing microcosm experiments on natural plankton assemblages from the Nueces Delta, TX, USA. On three occasions (March, June, and September 2001), experiments of semi-continuous and flow-through design were conducted using natural plankton assemblages. We investigated the effect of two different inflow and nutrient loading regimes on zooplankton biomass, and phytoplankton biomass and diversity, i.e., continuous and pulsed inflows of 3 day frequency. Despite differences in initial community structure on these three occasions, as well as the very different communities that arose between experimental designs, our findings showed that pulsed inflows altered plankton dynamics. In all cases, pulsed inflows resulted in greater zooplankton biomass. In most cases, pulsed inflows resulted in lower phytoplankton biomass and higher diversity. We speculate that greater phytoplankton diversity in the pulsed flow treatments favored selectively feeding zooplankton, whose better performance prevented higher accumulation of phytoplankton biomass.

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