Return of foundation species: effects of large-scale seagrass restoration on hard clam population dynamics and secondary production across spatiotemporal scales

Foundation-species theory posits that habitat-forming organisms enhance consumer productivity by physically or chemically reshaping their environment and, in so doing, controlling major ecosystem functions, such as primary or secondary production. Other ecological frameworks—scale-transition, Moran-effect, and stress-gradient theories—suggest that this facilitation may not be universal or may depend on scale: non-linear local effects can change direction when averaged over larger areas or longer periods, region-wide climate signals can synchronize populations and override habitat differences, and the balance between facilitation and competition can shift with abiotic stress, predator pressure, or life stage. I evaluated these competing expectations in the world’s largest restored seagrass meadow (exceeding 20 km²) on the Eastern Shore of Virginia, USA, pairing the foundational seagrass Zostera marina with a widespread and highly associated consumer, the hard clam Mercenaria mercenaria. Across a seascape that underwent a multidecadal transition from an unvegetated lagoon seafloor to a dense seagrass meadow, I combined centimeter-resolved phytoplankton profiling, seasonally replicated clam predation experiments, four-month clam growth assays, and a 45-year clam shell-growth chronology to quantify how seagrass habitat modification and regional climate interact across space, time, and life stage to affect hard clam secondary production and survivorship. Results reveal a driver hierarchy. Eelgrass canopies create a two-phase food pulse: a winter–spring reservoir of significant benthic (sediment) microalgal productivity followed by an eight- to ten-fold summer surge of near-bed suspended chlorophyll-a. The seagrass canopy––especially during spring and summer––significantly enhanced survival across life stages compared to unvegetated sites, with small newly-recruited individuals (~10 mm in length) benefitting most. While experimental observations of adult shell growth rates were marginally or entirely unexplained by variations in seagrass habitat or food availability, juvenile clams deployed at seagrass sites grew 36% more faster and were spared notable tissue mass reductions per unit of shell growth exhibited by those at bare sites, indicating an energetic tradeoff/cost lessened by seagrass. At decadal scales, March–August sea surface temperature anomalies alone explained about one-third of annual shell-growth variance, while proximity to the expanding meadow had a relatively small positive effect, placing M. mercenaria among the most climate-sensitive marine bivalves. Collectively, these findings show that intrinsic habitat mechanisms operate as predicted but only when dense habitat, life stage, and seasonal windows align, whereas regional climate imposes a dominant, lagoon-scale synchronizing influence on growth over interannual to multidecadal scales regardless of seagrass presence. Hence, the effect of this foundational seagrass species on hard clams appears to be dominated by effects on recruitment through provisioning a predation refuge to small, young individuals while productivity is largely governed by climate.