Experimental evidence for resilience of rockweeds on rocky shores in the Gulf of Maine, USA

Abstract

Resilience of ecological systems has been a central focus in aquatic sciences over the last 20 yr and critical to the understanding of ecological dynamics. Here, data from long-term time series and experimental manipulations are used to examine resilience of two fucoid macroalgae in the Gulf of Maine, USA. Experimental clearings that mimic ice scour of different sizes were established in stands of Ascophyllum nodosum in the winter of 1996–1997. Half of the clearings were scraped again during winter 2010–2011 to compare trajectories between plots that had been recleared and those that remained intact. Short-term experiments were done to test hypotheses of inhibition of recruitment and influence of hydrodynamic exposure, clearing size, and consumers as drivers of the observed successional trajectories. A. nodosum was resilient to small clearings within two generation times (15–18 yr), but resilience lessened with increased magnitude of perturbation leaving nearly half of large clearings dominated by another fucoid, Fucus vesiculosus after 25 yr. F. vesiculosus was resilient against invasion by A. nodosum when it recruited early and grew rapidly, dominating space within two to three generation times (~ 3 yr). When A. nodosum established a foothold prior to dominance by F. vesiculosus, F. vesiculosus was a long transient state. Water motion, lack of consumers, and large clearings favor rapid domination by F. vesiculosus, which inhibits establishment of A. nodosum, thus forming an alternative stable state. For A. nodosum, its longevity and large biomass per unit area are key to A. nodosum inhibiting F. vesiculosus and forming the other stable state.