Dissolved Inorganic Carbon (DIC) Preference and Seaweed Distribution on the Wellington South Coast

The distribution of seaweeds is determined in part by the interaction between the individual species’ physiology and the abiotic conditions of their environment. Dissolved inorganic carbon (DIC) acquisition is one such physiological process that is underexplored. Seaweeds acquire DIC from carbon dioxide (CO2) and bicarbonate (HCO3-). All seaweeds can passively absorb CO2 into their tissue through diffusion and facilitated by enzymatic processes. To obtain HCO3-, the seaweed must use an active process called a carbon concentrating mechanism (CCM) which uptakes the carbon molecule with pumps or symports. Though active uptake is common, not all seaweeds can utilise it. The distribution of seaweeds without a CCM is strongly determined by the CO2 concentration of the water in higher tidal zones. Seaweeds without a CCM are more common in deep, low light seawater with ambient CO2 concentrations. CCM seaweeds have a greater abundance overall in most ecosystems and are more common in shallow habitats with high light intensity. In the shallow subtidal and rockpool environment, light intensity increases in summer and the CO2 concentration decreases. CO2 concentrations can almost be negligible in highly productive rockpools, especially at low tide in summer. The goal of this paper was to test the following: 1) the difference in the abundance of seaweeds based on their CCM status between sites and seasons in the subtidal and rockpool habitats of the Wellington South Coast; and 2) the change in DIC use by CCM seaweeds between sites (subtidal n=3 transects, rock pool n=10 pools), summer and winter, and the subtidal and rock pool habitats on the Wellington South Coast. The surveys of the two habitats found a difference in seaweed community composition in the subtidal between seasons and sites. In the rockpool habitat, the community composition changed between sites, but not seasons. The community assemblages were largely typical of the two habitat types. CCM seaweeds were overall most common in the subtidal. Non-CCM seaweeds were not present in the rockpool habitat. However, there was no significant difference in the abundance of CCM seaweeds between seasons in either habitat. There was a greater abundance of CCM and non-CCM seaweeds at the Houghton Bay site in the subtidal habitat. Moa Point and Breaker Bay had the lowest abundance of CCM seaweeds in the rockpool habitat due to more bare rock. A significant difference was found in δ13C values between sites, seasons, and shore heights, but responses were highly species specific. Ulva spp. and Corallina spp. had more negative δ13C values in the subtidal area, indictive of greater CO2 reliance. However, the δ13C values of Carpophyllum maschalocarpum from the rockpools were surprisingly more negative than those in the subtidal habitat. This could be a life cycle-driven response, with more juvenile Carpophyllum maschalocarpum individuals in the rockpools. Corallina spp. had more negative δ13C values in winter, indicating more passive uptake as predicted. Some species also showed a significant difference in δ13C values between field sites. The δ13C values of Carpophyllum maschalocarpum, Corallina spp., Cystophora retroflexa, and Zonaria spp. were all significantly different depending on the field site. Overall, the subtidal and rockpool communities showed very little seasonal change based on DIC preference. There was much greater difference between sites than originally hypothesised, both in abundance by DIC-preference and in δ13C values. These responses are also species specific. A seaweed’s DIC-preference alone cannot determine how it will respond to changes in light and CO2 concentration.