Nutrient Loading Regime Determines N and P Limitation and Alters Ecosystem Function in Simulated Coastal Wetlands Along a Climate Change Gradient

Abstract:

Hydrologic and nutrient loading regimes in wetlands are influenced by regional climate, seasonal patterns, and upstream land cover and are important drivers of plant community structure and ecosystem function. Outlining the Laurentian Great Lakes, coastal wetlands receive nutrients from various upstream sources with loading rates that vary temporally and spatially with seasonal snowmelt, fluctuating lake levels, and land use practices. With this nutrient pulsing the ratio of nitrogen and phosphorus (N:P) also varies and receiving wetlands can flip between being N or P limiting. Shifting N:P ratios that result from changes in the magnitude and timing of nutrient inflow can have unpredictable effects on wetland ecosystem functions and nutrient cycling dynamics due to various complex ecosystem interactions. Here we investigate how wetland nutrient retention, transformation, and export differ between continuous press versus pulse nutrient loading regimes with anticipated changes in annual average temperature and growing season length (climate change scenarios RCP4.5 and RCP8.5 in years 2050 and 2100) using Mondrian, a community-ecosystem process-based model. In a factorial design, we test combinations of presses and pulses of high N, high P or both high N and P annual loading (g m^-2 y^-1) along temperature and growing season gradients. We predict that across plants with differing C:P and N:P target tissue ratios, plant tissue with high C:P target ratios will perform best when P is limiting, and plant tissue with high C:N ratios will perform best when N is limiting. We also predict that early-season pulses of nutrients will result in more nutrient export and less nutrient retention in wetlands than later pulses. Model results suggest that under the same annual nutrient load (g m^-2 y^-1), seasonal nutrient pulses result in more ecosystem productivity compared to press nutrient loading. However, shifts in temperature and growing season length associated with climate change have the largest effect on wetland function, including nutrient retention and transformation. We anticipate these results will help us better understand the phasing of springtime nutrient pulsing with the emergence of herbaceous wetland plants at a time when these phenomena are frequently changing.