Erosion Dynamics of Cultivated Kelp, Saccharina latissima, and Implications for Environmental Management and Carbon Sequestration

Abstract

A growing trend of interest for the cultivation of kelp is driven by predictions for high global demands of important commodities, which require the development of alternative supplies of natural resources. In this study the dynamics of loss of biomass from cultivated Saccharina latissima were studied from February to August 2018 at two kelp farms in Northern (69°45.26′N/019°02.18′E) and in Mid-Norway (63°42.28′N/08°52.23′E). Kelp fronds at each farm were individually followed throughout the growing season. Sectional regression was applied for conversion of measured frond lengths to estimated dry weights. The study shows that between 40 and 100% of all individuals in the studied kelp population constantly eroded slightly from their distal ends. However, until June the accumulated loss was only 8% of produced dry weight. Due to dislodgement of whole sporophytes this picture changed in July and August to heavy losses in Mid-Norway. Thus, the overall losses of kelp in terms of accumulated dry weight were only 8–13% of the gross growth until harvest in June in Mid-Norway and August in Northern Norway. Losses increased significantly in Mid-Norway during July and reached 49.4% of the annual production in August. The rates of losses were separated into specific erosion and dislodgement rates. Erosion rates over the whole experimental period for the two sites were not significantly different, while differences in dislodgement rates between farm sites proved to be highly significant. The exported annual amount of carbon was estimated on the basis of lost and measured carbon content in the tissue. From these data a scenario was built for a commercial Norwegian kelp farm growing S. latissima showing a carbon export of 63–88 g C m–2y–1. This is eight times less than has been reported from scenarios for kelp farms in China. This study confirms that optimal timing of harvest is the most important management tool for avoidance of heavy losses from kelp farms. In conclusion, an industry with early harvest will likely have a low carbon export, while a late-harvested bulk production could export four to six times as much carbon with an increased potential for carbon sequestration.

Dispersal and Deposition of Detritus From Kelp Cultivation

“…The transport of POM from the beginning of April until the end of June was considered, assuming deployment of the kelp cultures in January-February (Boreal winter). Before this period, the absolute biomass, and biomass export of a farm is low, even if the size and biomass specific growth rates usually are high (Forbord et al, 2020;Fieler et al, 2021). 4).…”

“…It was assumed that 10% of the total amount of POM was released in April, 20% in May, and 70% in June (Fieler et al, 2021). The release was assumed to be constant over each month, i.e., the same amount of matter was released for every model time step within each month.…”

“…Recent work suggests that the potential for macroalgae aquaculture globally is great, also outside of Asia, that currently produces more than 99% of the 32 million tons wet weight cultivated per annum (Lehahn et al, 2016;Hadley et al, 2018;van der Molen et al, 2018;Broch et al, 2019;FAO, 2020;Forbord et al, 2020;Aldridge et al, 2021;Duarte et al, 2021). During the grow out phase in culture, as in natural populations, macroalgal tissue fragments are shredded and entire plants dislodged (Parke, 1948;Sjtun, 1993;Krumhansl and Scheibling, 2012;Zhang et al, 2012;Pedersen et al, 2020;Fieler et al, 2021). Both these processes contribute to the pool and export of Particulate Organic Matter (POM).…”

“…The erosion of cultivated S. japonica in Sungo Bay in China has been reported at up to 61% of the cultivated kelp NPP (Zhang et al, 2012). Recent results from Norwegian kelp cultivation indicate a POM export of 8 to 13% of the NPP if the kelp is harvested early during spring, and up to 49% if harvested later in the growth season during summer (Fieler et al, 2021). The export includes a wide range of kelp fragment sizes (Fieler et al, 2021).…”

“…Recent results from Norwegian kelp cultivation indicate a POM export of 8 to 13% of the NPP if the kelp is harvested early during spring, and up to 49% if harvested later in the growth season during summer (Fieler et al, 2021). The export includes a wide range of kelp fragment sizes (Fieler et al, 2021). POM released from natural kelp forests is an important source of food and habitat for bacteria, macro-and meiofauna (Duggins et al, 1989;Renaud et al, 2015;Queirs et al, 2019;de Bettignies et al, 2020a;Brunet et al, 2021;Harbour et al, 2021a).…”