Temperature and salinity effects on submerged aquatic vegetation traits and susceptibility to grazing

Abstract

Submerged macrophyte vegetation provides valuable ecosystem services, but climate- and management-driven changes may alter plant traits in unexpected and interactive ways. Further, such changes in plant traits may influence herbivore response, with feedback to bed characteristics. We manipulated temperature (20, 25, 30 °C) and salinity (0, 6, 12) in mesocosms to simulate current and predicted scenarios for the San Francisco Bay area. We measured traits of Stuckenia pectinata (sago pondweed) and subsequent effects on invertebrate (amphipod, Ampithoe valida) grazing. Counter to predictions, higher temperatures tended to have positive effects on plant traits (leaf area, aboveground biomass, nitrogen [N], phosphorus [P], protein, and total phenolic content). Also, unexpectedly, the highest salinity treatment had few negative effects except when temperature was coolest; i.e., 20 °C and a salinity of 12 led to decreased carbon [C], N, P, protein, phenolic concentrations, and aboveground biomass. Conversely, the highest salinity (12) at the highest temperature (30 °C) produced the highest leaf N and P, and plants from this treatment suffered most from herbivory. Consumption rates significantly increased with lower leaf fiber and higher total leaf N and protein content; i.e., plants with more nutritious leaves and less structural defense were consumed most. Climate change is expected to increase both salinity and temperature, but manipulation of freshwater supply could lead to decreased salinity. The range of responses in S. pectinata traits and invertebrate grazing shown by our results imply that the specific combination and magnitude of human influences will differentially shape these submerged macrophyte beds and their functions.