Life history and morphological shifts in an intertidal seagrass following multiple disturbances

Abstract

Disturbance regimes differ in type, magnitude, and frequency, but few field experiments have considered compounded effects of disturbance. In this study, we characterized gap recovery after complete removal of eelgrass (Zostera marina) in Willapa Bay, Washington, USA. In separate experiments, we imposed two disturbance types – shoot damage and shoot removal – at different magnitudes and frequencies. We examined a broad suite of morphological and life history responses relevant to the ability of Z. marina to tolerate and recolonize following disturbance. We found that at multiple sites, 4m2 gaps in eelgrass meadows recovered in 2years. Experiments revealed that cut shoots had a short-term reduction in growth, decline in shoot width in relation to the number of cutting events, and a drop in clonal branching. In contrast, in thinned plots, both clonal branching and seedling size increased. Upper-elevation intertidal plants shared many traits of cut plants, particularly lower growth and size, but maintained similar clonal branching and shoot densities as at lower intertidal elevations. Sexual reproduction appeared inflexible to disturbances imposed over the summer experimental period but differed by elevation, with upper-elevation plants showing increased flowering, lower seedling densities, but higher seedling survival. Effects of multiple disturbances were generally additive even though individually their effects varied in magnitude and direction. One exception was an antagonism that appeared in sheath width: cutting had less effect at upper than lower elevation. Overall, Z. marina tolerated shoot damage through remodeling that reduced shoot size, and shoots compensated following loss of neighbors via enhanced clonal branching. Recolonization in this estuary should be further accelerated by sexual reproduction, despite weak responses to disturbance treatments, because seedling densities in late summer (4m−2) contributed 20–56% of shoot births.