Morphological response of Zostera marina reproductive shoots to fertilized porewater

•Fertilized reproductive Z. marina shoots had a significantly greater shoot height.•Fertilized flowers had greater numbers of rhipidia, spathes, and developing ovaries.•Nutrient enrichment advanced the stage of anthesis at sampling time point.

Exposing Zostera marina to fertilized porewater has been shown to increase vegetative shoot density and leaf growth rates, and affect shoot morphology. However, aside from changes in shoot density, records of morphological changes to reproductive shoots are lacking. To address this, five replicate ambient and nutrient enriched patches, each between 0.5 and 2.0 m2, were selected within a developing Z. marina meadow in Shinnecock Bay, NY. Fertilizer stakes, with a N:P:K of 15:3:3, were used to amend the sediments of a subset of patches in the late fall and early spring. In June, morphological measurements of reproductive shoots and stages of anthesis were recorded. We found significantly increased reproductive shoot height, number of rhipidia, and number of spathes on each rhipidium in response to fertilization. Nutrient enrichment also advanced the stage of ovary development in the first spathes at the time of sampling, indicating that the rate of development had been accelerated or the reproductive shoot had flowered earlier. Additionally, the number of normally developing ovaries, as determined by the size of the seed embryo, was significantly greater in enriched patches. We estimated that these changes acted to increase seed output per reproductive shoot, and may have enhanced pollen access by elevating receptive stigma above the local vegetative canopy.