COS 236-4 - Novel plant assemblages favored in waterfowl management boost plankton production in managed wetlands

Abstract: Working landscapes can provide key ecosystem services in altered landscapes for which restoration to a historic state is infeasible. In the case of heavily impacted ecosystems, such as estuaries, the loss of wetlands to homogenous and unproductive open water habitats is difficult to reverse. Our research explores the potential for actively managed wetlands to promote threatened estuarine functions. Managed wetlands are human-made diked, shallow ponds–with controlled hydroperiod and water exchange–that are found globally along coastal or estuarine environments, and are typically operated to promote waterfowl hunting, aquaculture, rice farms, or industry. Little is known about how managed wetlands influence aquatic food webs beyond directly harvested taxa, or how management can be optimized to benefit aquatic ecosystems.

Seasonally-flooded managed wetlands operated by duck hunting clubs in the San Francisco Estuary (California, USA) were recently shown to promote increased zooplankton production–a declining estuarine resource that is critical to imperiled native fishes. We hypothesize that plankton blooms in managed wetlands are driven by nutrients released from senescing plants after flood pulses, and that the effect on production varies with plant functional type. Duck club managers actively promote a novel assemblage of seed-bearing forbs to attract waterfowl to their wetlands, while emergent graminoids dominate in unmanaged tidal marsh habitats. We conducted a mesocosm experiment to test the effect of senescent plant material from three forb species and three emergent graminoid species on phytoplankton and zooplankton production using equal dry weights of material from plants collected in local managed wetlands.

Material from forbs increased plankton production because forbs are relatively labile when flooded, thereby releasing pulses of nutrients and carbon to fuel production. In contrast, emergent graminoids limited plankton production likely due to their ability to resorb nutrients into their rhizomes prior to senescence. Our results imply that managed wetlands are more functionally akin to floodplains–where productivity is driven by the flood pulse effect–than to parent tidal marsh habitats. Our findings demonstrate that beneficial functions and services once provided by historic ecosystem states can be replicated by new mechanisms under novel ecosystem states.