Sediment collection
Sediment samples were collected in 2016 at six marine finfish farms
located in BC, Canada: Baxter Islet (BI, May 10), Bedwell Sound (BS,
April 13), Midsummer Island (MI, June 8), Mussel Rock (MR, April 12),
Plover Point (PP, May 11), and Venture Point (VP, June 14). Samples were
collected at seven stations along a transect in a dominant current
direction initiated from the fish cage edge. The seven stations were
intended to be 0 m, 15 m, 30 m, 60 m, 90 m, 125 m, and 500 m from cage
edge. Samples were collected from those distances from cage edge where
possible; the exception was the reference station at MI, which was 2260
m from cage edge as it was the closest reference station found with soft
(and thus ‘grab-able’) sediments. Detailed site information was provided
in Table S1.
A total of three sediment grab samples were collected at each station
using a Van Veen Grab Sampler (0.1 m2, 24L)
(Blomqvist, 1991;
Gage & Bett, 2005). Pore-water sulphide concentration and redox
potential for each grab were measured prior to the collection of other
environmental variables to avoid oxygen injection into the sediment.
Sediments were classified into four different sulphide concentration
categories according to the Canadian Aquaculture Activities Regulations
(AAR), as follows: anoxic (> 4,500 µM), hypoxic (1,300 –
4,500 µM), oxic B (700 – 1,300 µM), and oxic A (< 700 µM)
(AAR, 2016). How
each station was sampled is presented schematically in Figure S1 and
summarized here. One grab sample was devoted entirely to macrofauna
analysis (details below); each of the other two grabs were used to
collect samples for geochemical analyses (porosity and organic content)
and three eDNA samples, which were collected from the top 2 cm of
sediment. The samples for geochemical analyses were collected using
spatulas and eDNA samples were collected using modified 60 cc
syringe-cores according to the sediment and meiofaunal methods detailed
in Sutherland et al .
(2007). Pore-water
sulphide concentration, redox potential, porosity and organic content
were measured following protocols used in Sutherland et al .
(2007). To avoid
cross-contamination during sample collection, the grab sampler was
rinsed in salt water and visually inspected for cleanliness between
grabs, samples were collected from the center of the grab only, and
syringes were cleaned using diluted bleach solution (final concentration
of 3% sodium hypochlorite) between farms. In summary, total numbers of
the various sample types collected from a total of 42 stations were: 42
benthic macrofauna samples; 84 samples measured for porosity and organic
content; 126 samples analyzed for sediment pore-water sulphide
concentration and redox potential; and 252 samples analyzed for eDNA
metabarcoding.