Loss of Spartina alterniflora genetic diversity associated with marsh bank fragmentation after long-term fertilization
Salt marsh loss is a major problem worldwide, due to associated losses of ecosystem services. For example, salt marshes serve as nursery grounds for many juvenile fish and invertebrates, while also filtering terrestrial nutrients and stabilizing shorelines. Anthropogenic nutrient pollution has been identified as a major factor contributing to salt marsh loss due to creek bank destabilization induced by changes in belowground allocation to roots that typically stabilize creekbanks. We hypothesized that nutrient-induced changes in ecosystem function in salt marshes are the net result of plastic or genetic responses of the foundation species, Spartina alterniflora. We took advantage of a long-term ecosystem-level nutrient enrichment experiment (TIDE) within the Plum Island Ecosystems LTER, MA, USA to examine the effect of eutrophication on the foundation salt marsh plant Spartina alterniflora. We analyzed the genetic composition and diversity of pre-and post-fertilization leaf samples using 9 microsatellite loci and also measured plant growth patterns in the field. Leaf samples were collected from transects in both nutrient enriched (10 or 5 years of treatment) and reference (10 and 5 year references) creeks. To evaluate the genetic composition prior to treatment, we also analyzed leaf samples that were collected and archived at the initiation of nutrient enrichment.
We found that S. alterniflora in the fertilized marshes were significantly less genetically diverse and the microsatellite genotypes present in the fertilized marshes were a subset of those present initially. In contrast, the unfertilized reference marshes had retained the full range of initial genotypes and genetic diversity. Over this same time period, we have observed widespread creekbank fragmentation and marsh collapse, which we previously attributed to changes in belowground allocation that compromised the function and stability of this ecosystem. We have also observed changes in growth patterns, growth rates, and flowering phenology that may be genetically inherited at these sites. Plastic changes may suggest rapid recovery when nutrients decrease, while genetic changes may prolong recovery. However, at this time, it is unclear whether functional differences in S. alterniflora growth patterns resulted from phenotypic plasticity or from selection by eutrophic conditions favoring a subset of genotypes. Future experiments are planned to disentangle the role of phenotypic plasticity from genetic filtering on salt marsh collapse.