Nurse plants modify the environment to facilitate the recruitment of propagules, and are potentially valuable tools for ecological restoration. Yet empirical tests, particularly in polluted environments, remain rare. The few studies that have examined nurse-effects in polluted environments report exclusively positive effects, but most tests have focused on pollution-tolerant species in metal contaminated environments. Biotic interactions are highly context-dependent, however, such that extrapolations to other suites of species and pollutant types appear premature. We examined changes in intraspecific nurse-effects across pollution regimes for a pollution-sensitive, macroalgal ecosystem engineer that is a target for intertidal restoration. In a manipulative field experiment, we out-planted propagules in the presence and absence of conspecific canopies at (i) unpolluted control shores and (ii) shores that received a partially-remediated (low toxicity) sewage effluent, and monitored the performance (survival and growth) of these propagules over-time. Algal canopies facilitated the survival and early growth of propagules at control sites but not at partially-remediated sites, where propagules performed poorly irrespective of canopy presence. Synthesis and applications:  Using a pollution-sensitive ecosystem engineer, we show for the first time that nurse-plant effects can be contingent on pollution regime, and that exposure to pollution (even at low levels) can erode nurse effects. We caution that nurse-plants are unlikely to be a universal tool for ecological restoration, even in systems where facilitative nurse-effects naturally occur (in our case, algal canopies on unpolluted rocky reefs). Management practitioners should carefully consider the pollution tolerance of target species, and test whether nurse-plant effects are maintained under current pollution regimes, before large-scale translocations are attempted.

Data from our field experiment that investigated changes in intraspecific nurse-plant effects across pollution regimes for the intertidal macroalgal ecosystem engineer, Hormosira banksii. We outplanted Hormosira propagules on experimental plates (5 x 5 cm) that were positioned beaneath a conspecific canopy (Canopy treatment) or without a canopy (Exposed treatment) at three sites adjacent to a partially-remediated sewage outfall (Partially-remediated sites; SH = Shelley Beach; O = Outfall; OE = Outfall East) and three unpolluted (Control) sites that did not recieve any sewage effluent (BY = Backyards; BT = Booties; C = The Craggs). We measured two performance metrics (survival and growth) of these outplanted propagules across two post-settlement times (4 and 12 days after outplanting). Survival was measured as the density of propagules remaining on the plates at each post-settlement time, which we quantified as the summed density of propagules within 25 haphazardly selected squares of a 25-cm² grid (i.e. values represent the number of propagules per 6.25 cm²). Propagule density was log (x +1) transformed for analysis. Growth was measured as the average size of propagules on the plates at each post-settlement time, which we estimated by measuring the widest diameter (in µm) of ~10 haphazardly selected propagules per plate where possible, or less than 10 propagules for plates where fewer propagules were available for measurement (due to high mortality). Missing values for propagule size in the dataset indicates that no propaugles were available for measurement from those plates. Note that each site initially recieved 10 plates per canopy treatment and post-settlement time combination (10 replicates x 2 canopy treatments x 2 post-settlement times x 6 sites = 240 plates total), but several plates were lost during the experiment, resulting in slight inbalances in replicates across treatment combinations and sites.