Water column (water quality) sampling
Water column (water quality) sampling
Selection of sampling stations
The same principles as described above, regarding sediment sampling, should dictate the selection of water column sampling stations. Station positions should reflect gradients of impact, normally aligned with the axis of predominant current and could consist of the following distances from the farm: 0, 25, 50, 100, 200m and 1000m. Unlike the footprint of fish farms that is often evident on the underlying sediments, the influence of aquaculture activities on surrounding water quality is generally quite small and measurable only within close proximity to the farm perimeter. It is noteworthy that in highly dispersive areas, with fast currents, we may limit water column sampling to 2 stations – one in the vicinity in the farm and one at reference site, outside zone B, because it is very difficult to detect a gradient in such highly dynamic environments.
Sampling dates should correspond to peak production and warmest water conditions in order to capture maximal impact conditions. Samples should be taken along a diel or tidal cycle (minimal sampling should include one at low water and another at high water in tidal regimes) to include extreme conditions, e.g. DO sag at end of the night. Optimally, should establish diel measurement of all variables at a minimum of 2 stations (e.g. one at the farm site and second at anticipated intermediate effect station) within the area of anticipated (from hydrographical data) effect and another at a distant reference station.
The water column will be sampled by CTD, and by bottle sampler at 3 depths; surface, mid-depth and near bottom and Secchi disk depth will be determined at all stations. In addition, the algal growth bioassay should be conducted at all possible study sites.
CTD vertical profiles at each station should include: salinity, temperature, sigma-t, dissolved oxygen, chlorophyll, PAR (photosynthetically active radiation), light transmission, turbidity (nephelometer).
Water samples will be collected using Niskin (or similar) bottles, and taken for dissolved nutrients (including: ammonia, nitrate, nitrite, phosphate, silicate, total N, total P) and for suspended particulate matter (including: total suspended solids (including POC, PON, POP) and chlorophyll (for CTD fluorescence ground-truthing). For shellfish farm sites, the phytoplankton community composition is extremely important and should be characterized, where possible.
