Food web complexity alters phytoplankton resistance and resilience to nutrient pulses in experimental ecosystems

Tyler James Butts , Robert A Johnson, Michael Weber, Grace Marie Wilkinson 

Food webs may mediate the resistance and resilience of ecosystems to disturbances driven by climate change. In aquatic ecosystems, greater food web complexity is theorized to increasing the resistance (longer response time) and resilience (shorter recovery time) of primary production to pulse disturbances, yet experimental evidence is limited. We simulated two storm-induced pulse disturbances by adding nutrients (~3%, ~5% increase in ambient concentrations) to three ponds with low, intermediate, and high food web complexity and compared them to reference ponds with matching food web structures. We evaluated primary production response time (resistance) and recovery time (resilience) following each nutrient pulse using a response detection algorithm and evaluated evidence of a critical transition with online dynamic linear modeling (resilience). The response threshold was never exceeded in the high complexity pond following either nutrient pulse whereas the threshold was exceeded in both the intermediate and low complexity ponds following the first pulse. There was evidence of a critical transition in the low complexity pond following the first pulse. After the second nutrient pulse, chlorophyll-a exceeded the response threshold again in both low and intermediate ponds, but the response was 12 days faster and the recovery 14 days longer in the low complexity pond. The intermediate pond was on track for a faster recovery time before the end of the experiment. We empirically show that greater food web complexity confers greater resistance and resilience of phytoplankton to repeated pulses of nutrient loading and may help buffer aquatic ecosystems against increasing and intensifying disturbances.