Relative importance of vascular plants and algal production in the food web of a Spartina-invaded salt marsh in the Yangtze River estuary

The trophic importance of microphytobenthos (MPB), phytoplankton, C ₃ vascular plants and invasive Spartina alterniflora in benthic and pelagic food webs was studied in Jiuduansha, a newly formed salt marsh in the Yangtze River estuary, using natural stable carbon and nitrogen isotopic analyses and IsoSource evaluation. MPB was found to be the major food source of meiofauna and important in the diets of macrofaunal consumers. Carbon derived from C₃ vascular plants significantly contributed to the nutrition of all planktonic copepods and supported planktivorous nekton, but played a minor role in the benthic food web. Although phytoplankton could be utilized by all the consumers, low production in this highly turbid estuary might restrict its relative importance. High trophic-level consumers could get carbon from ¹³C-enriched MPB and ¹³C-depleted C₃ plants via zoobenthos and zooplankton, respectively. Neither benthic nor pelagic animals fed exclusively on S. alterniflora except for a gastropod species in the present study, indicating a minor contribution of this invasive C₄ plant to the food web of the Jiuduansha salt marsh. In light of the degradation of salt marshes in the Yangtze River estuary, the rapid expansion of this invasive C₄ plant may alter the nutrient foundation of resident and migratory consumers and thus significantly impact the ecosystem there.