Effect of low salinity on the microscopic stages of Macrocystis pyrifera: an experimental approach for explaining changes in population dynamics on annual populations.

Seasonal monitoring of an annual Macrocystis pyrifera population in the Chilean northern Patagonia revealed significant fluctuations, including years of disappearance, linked to low-salinity events in two consecutive springs. To assess low salinity's effect on microscopic stages, sporophylls were collected from Yerbas Buenas Bay (41° 40' 20 '' S, 72° 39' 25'' W) on the spring 2023 (September); following sporulation, cultures were started under standardized laboratory conditions. Cultures were initially maintained at optimal salinity (30 psu) until reaching the gametophyte stage. Subsequently, treatments were applied, i.e. salinities of 18, 22, and 26 psu, with exposure times of 3, 7, and 14 days. Simultaneously, negative and positive controls (15 and 30 psu) were applied, with salinity held constant. After the exposure times, 30 psu was restored until the end of the experiment. Variables such as gametophyte density and size, oogonium density, and finally, density and size of embryonic sporophytes, were recorded every 5 days to calculate daily averages. The results indicated that exposure to lower salinities (15, 18, and 22 psu), regardless of exposure time, negatively affects growth of female and male gametophytes, resulting in delayed development and smaller final size. Additionally, gametogenesis was influenced by the duration of the lower salinity pulses, with inhibition during exposure and a rapid appearance of oogonia once optimal salinity conditions were restored. Ultimately, exposing the microscopic phases to salinities below their optimal range had adverse effects on the subsequent developmental stage, reducing the density and size of embryonic sporophytes in the cultures.