DOI: 10.5061/dryad.3j9kd51mp
title: Resilient consumers accelerate the plant decomposition in a naturally acidified seagrass ecosystem

description: Anthropogenic stressors are predicted to alter biodiversity and ecosystem
 functioning worldwide. However, scaling up from species to ecosystem
 responses poses a challenge, as species and functional groups can exhibit
 different capacities to adapt, acclimate, and compensate under changing
 environments. We used a naturally acidified seagrass ecosystem (the
 endemic Mediterranean Posidonia oceanica) as a model system to examine how
 ocean acidification (OA) modifies the community structure and functioning
 of plant detritivores, which play vital roles in the coastal nutrient
 cycling and food web dynamics. In seagrass beds associated with volcanic
 CO2 vents (Ischia, Italy), we quantified the effects of OA on seagrass
 decomposition by deploying litterbags in three distinct pH zones (i.e.,
 ambient, low, extreme low pH), which differed in the mean and variability
 of seawater pH. We replicated the study in two discrete vents for 117 days
 (litterbags sampled on day 5, 10, 28, 55, and 117). Acidification reduced
 seagrass detritivore richness and diversity through the loss of less
 abundant, pH-sensitive species but increased the abundance of the dominant
 detritivore (amphipod Gammarella fucicola). Such compensatory shifts in
 species abundance caused more than a three-fold increase in the total
 detritivore abundance in lower pH zones. These community changes were
 associated with increased consumption (52-112%) and decay of seagrass
 detritus (up to 67% faster decomposition rate for the slow-decaying,
 refractory detrital pool) under acidification. Seagrass detritus deployed
 in acidified zones showed increased N content and decreased C:N ratio,
 indicating that altered microbial activities under OA may have affected
 the decay process. The findings suggest that OA could restructure consumer
 assemblages and modify plant decomposition in blue carbon ecosystems,
 which may have important implications for carbon sequestration, nutrient
 recycling, and trophic transfer. Our study highlights the importance of
 within-community response variability and compensatory processes in
 modulating ecosystem functions under extreme global change scenarios.
description: <strong>Study sites and environmental
 parameters</strong> We utilized two discrete CO2
 vents near the Castello Aragonese islet at Ischia Island (Italy,
 40°43'57" N, 13°57'52" E) (Fig. 2A). The first site
 was located at 2-3 m depth on the north side of the islet (referred
 henceforth to as North Castello), characterized by sloping rocky reefs and
 continuous tracts of P. oceanica beds surrounding the reef (Fig. 2B). The
 second site was located approximately 500 m away from North Castello and
 150 m from the nearest shore at a depth of 3-6 m (referred to henceforth
 as Vullatura). This site was characterized by soft-bottom habitats (sandy
 or dead <em>Posidonia</em> matte) surrounded by seagrass
 patches growing over 2-3 m tall pinnacles and ridges of
 <em>Posidonia</em> matte (Gambi et al., 2020; Mecca et al.,
 2020) (Fig. 2C). At North Castello, we established three pH zones
 (ambient, low, and extreme low pH; depth 3-3.5 m) delineated by previous
 studies based on seawater carbonate chemistry analyses and in situ pH
 monitoring (Hall-Spencer et al., 2008; Kroeker et al., 2011). At
 Vullatura, we established two study zones (low and extreme low pH) where
 the intensity of CO2 bubbling was comparable to that of acidified zones in
 North Castello. The ambient zone for Vullatura was established
 approximately 200 m away from the other study zones, at similar depth and
 surroundings (5.5-6 m) (Fig. 2A). We monitored the in situ seawater pH and
 temperature using autonomous sensors (Satlantic
 seaFET<sup>TM</sup> Ocean pH sensor). We simultaneously
 deployed two to three individual sensors in different pH zones and rotated
 these sensors around all six zones from June to October 2017.
  <strong>Effects of ocean acidification on detritivory and
 seagrass decomposition rate</strong> We conducted
 an in situ decomposition experiment by deploying litterbags containing
 seagrass leaf detritus. During May 2017, live
 <em>Posidonia</em> shoots (including aged and young green
 leaves) were collected from 3-5 m depth at San Pietro point &gt; 2 km
 away from the study vents and accessible from the Ischia Marine Center of
 the Stazione Zoologica Anton Dohrn (40°44'48" N,
 13°56'40" E). In the laboratory, we thoroughly rinsed all
 collected seagrass leaves in a freshwater bath to remove excess epiphytes
 and epifauna. Seagrass leaves were then dried at 60°C to constant mass. We
 selected leaves with a minimum amount of epiphytic materials and
 haphazardly grouped selected leaf fragments into 10 g aliquots. We
 constructed litterbags (20 cm × 20 cm) out of fiberglass insect screen
 with 2 mm openings to minimize the loss of detritus fragments without
 limiting detritivore colonization. We randomly assigned litterbags into
 one of six groups (n = 48 per group) for deployments in each of the six
 study zones. All litterbags were labeled and received a clump of
 pre-weighed aliquots of leaf fragments. In early June
 2017, we deployed a total of 288 litterbags in the field (Fig. 2C) via
 SCUBA. In each zone, we chose relatively flat seafloor surfaces composed
 mainly of dead <em>Posidonia</em> matte at similar distances
 away from existing seagrass beds. The deployment area was directly
 adjacent (&lt; 3-4 m) to the position in which pH sensors were
 anchored to the bottom. The water depth differed between North Castello
 and Vullatura, with the former vent sitting at shallower depths close to
 the shoreline, but within each vent site, we selected areas with similar
 depth at 3-3.5 m and 5.5-6 m, respectively. Prior to the deployment, we
 haphazardly grouped litterbags into clumps of four and firmly anchored
 these clumps to the bottom using aluminum and steel rods.
 Litterbags were retrieved after 5, 10, 28, 55, and 117 days of
 deployment. On each retrieval day, we collected eight litterbags per study
 zone (16 litterbags per zone on the last sampling date) and carefully
 enclosed each litterbag in a plastic bag. All bags were placed on ice
 until being transported to the laboratory. In the laboratory, we gently
 washed all litterbags with seawater and removed seagrass detritus and
 animals within. Seagrass detritus was thoroughly rinsed to remove all
 animals, sediments, and other organic materials. On the day-55 collection,
 seagrass leaves began showing numerous feeding marks and a greater extent
 of mechanical fragmentation. To quantify the relative intensity of
 detritivore feeding and mechanical fragmentation, we counted the number of
 all seagrass leaf fragments &gt; 1 cm in the widest length that had
 visible bite marks (referred to henceforth as the “Detritivory Index”).
 After these initial processing steps, seagrass detritus was dried at 60°C
 to constant mass. We analyzed a subset of dried seagrass samples (n = 8
 per study zone per date for sampling day 10, 55, and 117) for organic
 carbon and nitrogen content using a Carlo Erba CN Elemental Analyzer.
 Animal components from the samples were collected using a fine sieve (400
 μm) and preserved in 4% formalin. <strong>Effects
 of ocean acidification on seagrass detritivore community
 structure</strong> To identify and enumerate
 seagrass detritivores, we randomly chose four litterbag samples per study
 zone per sampling date (day 28, 55, and 117). <em>Posidonia
 detritus</em> is colonized by various invertebrates, but most of
 these species do not directly consume seagrass leaves and instead function
 as predators, grazers, scavengers, suspension/deposit feeders, or
 detritivores of algal materials (Remy et al., 2018; Gallmetzer et al.,
 2005). We, therefore, focused on species known to directly ingest dead
 seagrass leaves. We counted the gammarid amphipod <em>Gammarella
 fucicola</em>, which are highly abundant and important shredders
 of<em> P. oceanica</em> detritus (Remy et al., 2018; Lepoint
 et al., 2006; Wittmann et al., 1981). We also identified and counted other
 seagrass detritivores, including non-amphipod crustaceans (isopod
 <em>Zenobiana prismatica</em> and decapod Galathea
 <em>bolivari)</em>, snails (<em>Bittium
 reticulatum</em>), and urchins (<em>Psammechinus
 microtuberculatus</em>). It has not been confirmed whether
 <em>G. bolivar</em>i actively consumes seagrass detritus.
 However, considering the species is closely related and ecologically
 similar to Galathea intermedia, which occupies the same habitat (i.e.,
 seagrass litter) and consumes <em>Posidonia</em> detritus
 (Remy et al., 2018), we classified it as a potential seagrass
 detritivore.