52 DECEMBER 2017 • WORLD AQUACULTURE • WWW.WAS.ORG Environmental Sampling/Monitoring We completed two seasons (spring-summer 2011-2012) of environmental sampling and monitoring to gather data on water quality and physical changes associated with farm operations. This consisted of 1) placement of YSI 6600 water quality sondes 10 cm off the bottom at the center of the farm and at an adjacent seagrass reference location; 2) deployment of a current meter1 at the center of the farm to record water movements just above the sediment; 3) placement of Onset temperature data loggers at the northern end of the farm; 4) collection of discrete surface water samples for nutrients, chlorophyll a, particulate organic carbon (POC), total nitrogen (TN), and total suspended solids (TSS) analyses; 5) collection of macroalgae from the surfaces of unswept nets; 6) quadrant sampling to determine Manila clam densities and size ranges; 7) benthic infauna sampling from mechanically and manually harvested tracts and from an off-farm reference site; 8) epibenthic sampling from swept and unswept nets, and from sand and seagrass covered reference sites; and 9) fixed video observations of the harvested tracts, open and netted (unharvested) and seagrass habitats. All sampling was coordinated closely with farm operations and included an evaluation of mechanical harvest and an assessment of comparative manual harvests with traditional short-handled rakes. This information was also used in farm-scale modeling efforts described in Saurel et al. (2015). Water Quality Tidal current velocities at the center and edge of the farm were generally moderate and reached 25 cm/sec during peak tidal flow. Outgoing tides flowed to the north (350o) while incoming tides flowed to the south (170o). Velocities fell to 0 cm/sec at slack tide. Average water temperature ranged from 13.5 to 16.7 C. Exposure of the farm during low tide events resulted in shortterm peaks to nearly 30 C. There was little difference in pH and dissolved oxygen concentrations (2011 data) between the center of the farm and an adjacent unfarmed seagrass bed. Peak pH and dissolved oxygen values coincided with peak tidal velocities (25 cm/sec), regardless of water flow direction. Dissolved oxygen and pH also generally increased during the day from photosynthesis by phytoplankton and macroalgae. Water quality findings from multiple discrete samples taken on four sampling dates in 2011 are shown in Table 1. All samples were collected during an ebb tide as water left shallow intertidal flats and moved offshore across the length of the farm. • There was no consistent pattern of chlorophyll, dissolved carbon and nitrogen as water passed through the farm. Chlorophyll ranged from 1.5 to 5.8 µg/L, generally within the range of surface waters in other areas of north Puget Sound (www.ecy.wa.gov). There was a modest increase of carbon as summer progressed into fall, linked to increased TSS. • TSS concentrations did not decrease or decrease consistently in the late spring and early summer inside the clam farm as compared to water entering the site from the adjacent tide flat. However, there was a marked increase in TSS in the late summer and fall in water entering and exiting the farm, apparently the result of seasonal deterioration of Ulva and other macroalgae and of Zostera marina in the bay. • There were strong seasonal trends in nutrients, with the exception of phosphate. There were no consistent differences between sample sites for phosphate, silicate, and nitrite+nitrate. Silicate levels fell in the late summer and fall at each sample site, likely reflecting a decline in diatoms relative to other phytoplankton taxa. Levels of ammonium increased during the same period, exceeding levels reported during late spring and early summer. Water quality changes as tidal water flowed across the farm site were modest and seen in changed TSS and ammonium. Oxygen, carbon dioxide, ammonium, reactive silica and phosphorus fluxes in Sacca di Goro, an intensively cultivated lagoon in Italy, were stimulated several fold from respiration and excretion by clams (Bartoli et al. 2001). In this clam farm, oxygen consumption was 3 to 4 times and ammonium efflux was 1.9 to 4.9 times greater than those measured in a control site, with rates positively correlated with clam biomass (Nizzoli et al. 2007). Mesocosm and field studies suggest that bivalves are a major contributor of ammonium to intertidal water (Bendell et al. 2014). We observed an increase and a decrease in ammonium values in the water column within or adjacent to Chuckanut farm. Ammonium appeared to be driven by levels in water entering the farm from adjacent tide flats and not the increased presence of Manila clams. Biofouling Increased late spring to summer biofouling on predator nets by macroalgae (Table 2) can be attributed to increased bay temperatures and elevated ammonium concentrations at the sediment-water interface. FARM model results from the Chuckanut farm (Saurel et al. 2015) indicate that ammonium excretion by clams is a nutrient source for macroalgae. Further, planted rows with larger second and third year clams had greater seaweed biomass peaks. Higher density and larger clams have the potential to drive benthic metabolism in farmed areas and to sustain macroalgal growth through regeneration of inorganic N (Nizzoli et al. 2007). The physical presence of predator nets also provides favorable habitat for macroalgae colonization, which would normally be less likely to successfully colonize open sand sediment. The total biomass was substantial, calculated at 17.5 t wet weight in June 2011 and close to 29 t in early May of 2012 before intensive net sweeping (Table 2). By October of both years it was reduced to 2 to 3 t because of the combined effects of sweeping, decreased temperatures and decreased sunlight. Currently, swept seaweeds float away or decompose in the areas between clam rows, but Chuckanut is working on potential uses of swept macroalgae. Aquatic Life Around Chuckanut Farm The Chuckanut farm harbors a diverse assemblage of animals and plants living with and adjacent to clams, growing on predator nets, and swimming over the farm. These organisms were the objects of an intensive multi-season sampling effort addressing four principal areas: 1) harvest method: differences in benthic or in-sediment samples before and after harvest on mechanically and manually harvested plots; 2) overall farm effect: benthic samples from farmed plots compared to non-farmed plots; 3) net sweeping effect: epibenthic invertebrates from two different unfarmed seagrass and sand substrates compared with swept and non-swept predator nets; and 4) fixed video observations of fish and macrofauna at harvest, netted and reference sites.
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