Metal content of kelp (Laminaria digitata) co-cultivated with Atlantic salmon in an Integrated Multi-Trophic Aquaculture system

Aquaculture is the most rapidly expanding food production sector and this trend is projected to continue in the coming decades. Intensification of farming methods alongside a tendency for farming higher trophic level species has led to concerns over nutrient loading of coastal waters. Where this is the case, Integrated Multi-Trophic Aquaculture (IMTA) has been proposed as a best-practice management option for reducing environmental impact. Fed aquaculture can also affect metal concentrations of surrounding biota by acting as a point source of contamination or by altering the abiotic environment in a manner that affects the capacity of organisms to accumulate metals. Seaweeds are integral to IMTA in providing the inorganic nutrient extraction component of the system, however it is unknown whether their close proximity to other aquaculture operations facilitates increased metal accumulation. As a result, the effect of cultivation and growth environment on accumulation of metals in the kelp Laminaria digitata (Hudson) J.V. Lamouroux was investigated, providing the first comparative experimental data examining the effect of multi-species cultivation on trace metal content of macroalgae. Thalli of L. digitata growing wild, cultivated as a single species (mono-cultivated), and cultivated alongside organic Irish salmon aquaculture facilities (IMTA) were analysed for metal content (As, Ba, Cd, Co, Cu, Mn, Sb, V, Zn). Cultivation in an IMTA context raised the content of Cu, Mn and V relative to that in mono-cultivated seaweeds however concentrations of metals were within the range of those from algae collected from undisturbed wild populations. Metal levels in the seaweeds do not pose a concern over inclusion as a dietary component with the possible exception of arsenic that exceeded some legislative limits. The results of this study provide novel information that can inform policy and sustainable aquaculture management practices aimed at reducing contamination of the aquatic environment and the impact on consumers.This study provides the first comparative investigation of metals in seaweed from different cultivation environments and can be used to support best-practice management decisions involving IMTA. Laminaria digitata metal content was marginally increased for copper, manganese and vanadium at IMTA sites. Vanadium and antimony (emerging contaminants) in the commercially important seaweed L. digitata are reported for the first time.