Pathways to Spatial Subsidies by Kelp in Seagrass Meadows
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SPECIAL ISSUE: SEAGRASSES TRIBUTE TO SUSAN WILLIAMS
Pathways to Spatial Subsidies by Kelp in Seagrass Meadows Audrey E. Cartraud 1
&
Paul S. Lavery 1 & Caitlin M. Rae 1 & Glenn A. Hyndes 1
Received: 28 April 2020 / Revised: 28 September 2020 / Accepted: 22 October 2020 # Coastal and Estuarine Research Federation 2020
Abstract Spatial subsidies often occur from highly productive donor ecosystems to recipient systems with low productivity, but little is known about subsidies between highly productive ecosystems. Here, we investigated whether the kelp Ecklonia radiata, which is frequently dislodged from reefs, provides spatial subsidies in adjacent seagrass meadow where in situ productivity is already high. We clarified the pathways of the allochthonous kelp as a spatial subsidy by adding 15N-enriched kelp to Posidonia sinuosa seagrass meadows, tracing the uptake of kelp-derived nitrogen by seagrass epiphytes and epifauna. We observed a 32.5–52.2% loss of kelp biomass (wet weight), representing almost 13,000 μg of 15N from the labelled kelp. This corresponded with an increase in δ15N and 15N biomass of detritivores and epiphytes, showing either a direct or indirect (via epiphytes) assimilation of kelp-derived nutrients. The densities and biomass of the gastropod Strigosella lepidus and shrimps were higher in seagrass plots with input of E. radiata, but not for amphipods or the biomass of epiphytes. We estimate that the detritivores and epiphytes in the seagrass plots assimilated about 3% of the 15N released from the labelled kelp, and based on a data from previous study, the seagrass itself could have assimilated up to a further 6% of the released kelp 15N. Our findings show that in a highly productive seagrass ecosystem, where autochthonous food sources are available, the input of allochthonous resources such as kelp can supplement those resources and appears to play an important a role as a vector for a spatial subsidy. Keywords Epiphytes . Food webs . Seagrass . δ15N . Gastropods . Amphipods
Introduction Ecosystem boundaries are regularly crossed by materials, organisms, and energy, which can change the distributions, abundances, and growth rates of consumers at multiple trophic levels in recipient systems, thereby altering food web structures and community dynamics (Polis et al. 1997). This process, known as spatial subsidy, can produce terrestrial-aquatic/ Highlights • The input of kelp in seagrass meadows increased the densities and biomass of gastropods and shrimps. • Epifauna and epiphytes showed high uptake of 15N from allochthonous kelp. • Allochthonous kelp play an important a role as a vector for a spatial subsidy in seagrass meadows. Communicated by Masahiro Nakaoka * Audrey E. Cartraud [email protected] 1
Centre for Marine Ecosystems Research, School of Science, Edith Cowan University, 270 Joondalup Dr, Joondalup, WA 6027, Australia
marine, aquatic/marine-terrestrial, and marine-marine linkages (Duggins et al. 1989; Polis et al. 1997; Hyndes et al. 2014; Larsen et al. 2016), particularl
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