Seaweeds take up dissolved nitrogen and phosphorus as they grow, and harvesting removes those nutrients from the water body. This is the best established of the benefits. It underlies integrated multi-trophic aquaculture, in which seaweeds are grown beside fed fish or shrimp so that the waste of one crop becomes the input of another (Chopin et al., 2001; Neori et al., 2004). At a regional scale, the very large seaweed harvest of China has been estimated to remove a meaningful fraction of the nutrients entering its coastal seas (Xiao et al., 2017). The same process has a less welcome side in nutrient-poor waters, where a dense farm competes with phytoplankton and natural vegetation for a limited supply (Campbell et al., 2019).

Suspended lines of seaweed add three-dimensional structure to open water, and fish and mobile invertebrates gather around them. Whether this represents new production or the attraction of animals from nearby habitats is seldom resolved, and the structure is removed at every harvest. Reviews of the environmental effects of farming treat the habitat benefit as plausible but poorly quantified (Campbell et al., 2019).

The carbon argument has two parts. Wild macroalgae export a share of their production to deep water and sediments, where some of it is stored for long periods (Krause-Jensen and Duarte, 2016), and farmed seaweed may do likewise through fragments shed before harvest. In addition, seaweed products may displace materials with a larger carbon footprint (Duarte et al., 2017). Modelling suggests that offsetting on a scale relevant to national emissions would require farming over very large areas (Froehlich et al., 2019).

The accounting has been challenged. Hurd et al. (2022) point out that carbon fixed by a seaweed is not the same as carbon dioxide removed from the atmosphere: the air-sea exchange that completes the removal is slow, calcifying organisms on the fronds release carbon dioxide, and most harvested biomass is consumed and returns to the atmosphere within a short time. On present evidence, a small farm should not be described to its owners as a carbon sink, and income from carbon credits is not a sound basis for a household's planning.

Tropical carrageenophytes are propagated by cuttings, so a farming region may be planted with a very small number of clones. The crop has been domesticated at great speed with little of the breeding and health management that accompany land crops (Loureiro et al., 2015). Two problems recur. Ice-ice disease, a whitening and softening of the thallus that follows stress from high temperature, low salinity or low light and is aggravated by opportunistic bacteria, can destroy a crop within days (Largo et al., 1995). Infestation by filamentous red algal epiphytes lowers yield and carrageenan quality and has spread through the main producing countries (Vairappan et al., 2008). In Tanzania, repeated die-offs of the higher-value Kappaphycus have pushed farmers towards the hardier but cheaper Eucheuma denticulatum (Valderrama et al., 2013). A policy brief on the future of the industry identifies weak biosecurity, including the unregulated movement of seed stock between countries, as its principal vulnerability (Cottier-Cook et al., 2016).

Kappaphycus and Eucheuma are native to South-East Asia and have been carried to many other tropical coasts for farming. In some places they have left the farms. In Kane'ohe Bay, Hawai'i, Kappaphycus introduced for research and cultivation spread across reef habitat and overgrew coral, and proved difficult to remove (Conklin and Smith, 2005). Overgrowth of live coral by Kappaphycus alvarezii has also been reported from the Gulf of Mannar in southern India (Chandrasekaran et al., 2008). Establishment is not inevitable, and in many introductions no spread has been recorded, but the outcome is hard to predict in advance and expensive to reverse. For temperate kelps the parallel concern is genetic: cultivated strains may interbreed with wild populations nearby (Campbell et al., 2019).

Off-bottom farms are usually sited on shallow sand flats, which in the tropics are often seagrass meadows. In Zanzibar, seagrass beds beneath farms had lower seagrass cover and different macrofaunal assemblages than comparable beds without farms, an effect attributed to shading, to the clearing of vegetation by farmers and to trampling during daily tending (Eklöf et al., 2005). Seagrass meadows are nursery grounds for the fish and invertebrates on which the same communities depend, so a loss here is set directly against the gain in farm income. European assessments list benthic shading, altered water flow and the deposition of organic matter among the effects that need study before farming is scaled up (Campbell et al., 2019).

Dried carrageenophyte is an industrial raw material sold into a chain that ends with a small number of processors. Farm-gate prices have risen and fallen sharply over the years, and farmers in remote villages commonly sell to a single buyer who also supplies their ropes and seed on credit. The six-country comparison found wide differences in the price farmers received and in their share of the export value (Valderrama et al., 2013). In Zanzibar, Fröcklin et al. (2012) concluded that the income earned by women farmers was low in relation to the hours worked and questioned whether the activity deserved its reputation as a development success.

The same Zanzibar study recorded that seaweed farmers reported poorer general health than women in other occupations, with complaints that included musculoskeletal pain, fatigue, eye and skin irritation and injuries from marine organisms, consistent with long hours bent over in salt water under strong sun (Fröcklin et al., 2012). These burdens seldom appear in appraisals of farming projects. For seaweeds grown as food there is a separate question of product safety: some species accumulate iodine, arsenic and other elements to levels that are subject to regulation in importing countries (Holdt and Kraan, 2011), and small producers are poorly placed to pay for testing.

Conservation programmes have often introduced seaweed farming as an alternative livelihood in the expectation that people who farm will fish less. Field studies in the Philippines and Indonesia found little support for this. Households tended to add farming to fishing instead of substituting one for the other, and income from seaweed was sometimes invested in fishing gear (Sievanen et al., 2005). In the central Philippines the number of fishers did not decline in villages where farming grew (Hill et al., 2012). Farming may still be worth supporting for the income it brings, but it should not be counted on as a fisheries management measure.