Trophic linkages across seascapes: Subtidal predators limit effective mussel recruitment in rocky intertidal communities

Food-web linkages between habitats across a landscape are known to be important in terrestrial systems. Linkages through the lower (nutrients) and higher (predators) trophic levels are also starting to be recognized across seascapes. Here, we test the potential importance of a subtidal-intertidal trophic linkage through predation on mussel recruits by fish in mussel-dominated communities at 2 tidal levels on wave-exposed shores of southern New Zealand. We show that on rocky shores that include both intertidal benches and subtidal reefs, labrid fish move into the intertidal zone during high tide and exert fast and strong predation pressure on small mussels in the low-, and to a lesser extent mid-shore, intertidal levels. Within 1 d, 80 to 95% of small (5 to 15 mm) transplanted mussels were removed from experimental tiles on the low shore when unprotected from fish predation, while 20 to 60% were removed on the mid-shore; in contrast, most fully protected mussels survived for months. Although at a slower pace, predation was also evident in cages that excluded fish but allowed access to large invertebrate predators. This predation occurred mostly at night when large predatory crabs were abundant at the study sites. The combined predation pressure on small mussels by fish and crabs was reflected in the scarcity of larger/older mussel recruits (5 to 15 mm) outside mature beds on the low shore compared to greater abundance on the mid-shore. The number of mussel settlers in artificial collectors and of smaller/younger recruits (<5 mm) within barnacle patches did not significantly differ between shore heights; thus, we attribute the difference in abundance of late recruits to differential predation rates by subtidal predators. This study clearly demonstrates that highly mobile subtidal predators, particularly fish, can have important impacts on temperate intertidal food webs, especially on early life stages of habitat-dominating invertebrates, such as mussels in New Zealand and probably elsewhere.