Nutrient inputs often promote changes in primary productivity that consequently affect associated food webs, and excessive inputs can destabilize ecosystems. How nutrient inputs are delivered to natural ecosystems (amount and duration of inputs) can affect community structure, but the effects of these variables have not been tested synchronously across multiple trophic levels for an entire community over time. We tested the impacts of three different levels of nutrient pulses versus presses on plants and arthropods across trophic levels to test whether trophic‐level abundance and biomass respond similarly to nutrient subsidies. We found that knowing how a short‐term pulse affects trophic abundance and biomass did not predict how a long‐term press will affect the same food web. Different trophic levels responded to fertilization level and duration treatments in different ways and these responses were often inconsistent across years. Notably, abundance and biomass were not good approximations for each other for plants; biomass responded positively to fertilization, but not plant abundance, resulting in fewer, larger plants. Surprisingly, abundance and biomass were good approximations for each other for arthropods, even though the biomasses of some of our individuals could vary 100–1000× within a single species. Arthropod responses in the first year of the experiment seem to be driven more by plant biomass and C:N ratios (plant quality) rather than plant abundance, which was unaffected by fertilization. Herbivores generally demonstrated no or a negative response to fertilization for both abundance and biomass, which may be due to the strong, positive response of predators to fertilization. Epigeic feeders initially demonstrated strong, positive responses to fertilization, but in the fourth year of the experiment, their abundance and biomass only differed from the control for the high press treatment. We found differences among pulses of the same magnitude across years and that the timing and frequency of subsidies can have as large an impact on communities as total nutrient amount. The subsidies we used in this experiment were conservative relative to nutrient inputs that marshes experience along the US Atlantic coast, yet led to significant, non‐additive effects on plants and the arthropod food web.
Murphy et al. (Thu,) studied this question.