The important role of vegetated ecosystems in the sequestration of carbon has gained strong interest across a wide variety of disciplines. With evidence growing of the potential for macroalgae ecosystems to capture carbon, there is burgeoning interest in applying newfound knowledge of carbon capture rates to better understand the potential for carbon sequestration. Seaweed farms are expected to play a significant role in carbon capture; advocates for the expansion of seaweed farms are increasing in many countries. In general, seaweed farms are expected to be highly productive, although whether they are autotrophic or heterotrophic ecosystems and hence potential exporters of carbon, is under debate. Therefore, we present our investigation of three seaweed farms, two in northern Japan and one in southern Japan. We examine the frequency of autotrophic days and compare potential rates of carbon capture of the seaweed farms with two natural macroalgae ecosystems and one degraded site. We estimated potential carbon capture rates by calculating the net ecosystem productivity from continuous recordings of dissolved oxygen concentrations under natural environmental conditions. The net ecosystem production rates for the natural ecosystems in Arikawa Bay and Omura Bay were equivalent to 0.043 and 0.054 [g C m-2 d-1] m-1, respectively. Whereas, for the degraded ecosystem in Tainoura Bay, it was -0.01 [g C m-2 d-1] m-1. We reveal that the Undaria pinnatifida farm in Matsushima Bay experience autotrophy more often than natural ecosystems, although for seaweed farms producing U. pinnatifida in Hirota Bay and Cladospihon okamuranus at Bise Point, autotrophy was less frequently observed. Nevertheless, up to 14.1 g C m-2 (0.110 g C m-2 d-1) was captured by the production of U. pinnatifida and 3.6 g C m-2 (0.034 g C m-2 d-1) was captured by C. okamuranus, and the total yield of carbon captured during 2021 production season for these farms was 43,385 kg C.
Global changes in climatic conditions are expected to disrupt marine ecosystems. Ocean warming is one of many concerns, since more than 90% of the Earth's warming occurs in the oceans. Macrophyte-dominated communities recently have become the focus of climate mitigation due to their high carbon sequestration rate. Therefore, there is an urgent need to understand the effects of environmental variables on the phenological response of photosynthesis in entire macrophyte communities (i.e., community production). We conducted 30 monitoring surveys from May 2015 to February 2017 and collected time-series data of environmental variables in Zostera marina (3 m depth) and Sargassum siliquastrum (1 m depth) communities. The community production and respiration from two different macrophyte communities were calculated from dissolved oxygen time-series. Analysis of the time-series indicated strong diurnal frequencies for dissolved oxygen, light, and net ecosystem production, whereas weekly frequencies dominated for water temperature, chlorophyll a fluorescence, and current speed. Water temperature appeared to drive mean gross ecosystem production over the course of a year and light induced variations in the short-term and were similar in both macrophyte communities.
We present a descriptive account of the dynamics of epiphytic diatoms, epifauna, and the leaf surface area of Zostera marina in a shallow water ecosystem. We hypothesized that the growth stage of the host macrophyte (i.e., leaf surface area) influenced the presence of epiflora and epifauna, as well as that the leaf surface area and epifaunal population density affected the cell density and species composition of epiphytic diatoms. To evaluate this hypothesis, we quantified the leaf surface area of a host macrophyte (Zostera marina), the presence of epifauna, and the community of epiphytic diatoms that could be observed on the leaves of Z. marina during the period from May 2017 to December 2018. We conducted a descriptive analysis of the time-series observations of leaf surface area, epiphytic diatom density, and epifauna population density. Epiphytic diatom density was low and epifauna density was high during the growing season of Z. marina. Epiphytic diatom density was high and epifauna density was low during the maturation and senescence periods of Z. marina. Our analysis shows that epifauna densities lagged epiflora densities by at least four months, and that epiflora densities lagged leaf area by four months. Therefore, we hypothesized that herbivorous gastropods and amphipods could alter species composition via their preference of food items (active choice) or by ingesting more of the species that were structurally more available (passive preference).
We artificially reproduced a scenario where the phytoplankton community changes under increased regeneration of nutrients from the sediment due to anoxic conditions using a mesocosm in Katagami Bay, one of many smaller bays in Omura Bay. Two instrumented mesocosms were used in the field experiment, whereas the surrounding waters acted as a control. The plastic bags used in the first mesocosm (M1) was completely transparent, whereas the second mesocosm (M2) was designed so that the bottom two-thirds of the plastic bag was opaque and only the top third was transparent. Photon flux density of M2 was about the same value in the surface layer as compared with M1 and Control, but it was approximately 1/2 in the middle layer and approximately 1/10 in the bottom layer. In M2, hypoxic water mass appears in the bottom layer on day 2 and then dissolved inorganic phosphate increased from day 3 to end of the experiment. At the start of the experiment, a diatom (Dactyliosolen flagilismus) was dominant in all compartments. Diatoms continued to dominate in M1 and the control, but in M2, a dinoflagellate (Heterocapsa circularisquama) became dominant at the end of the experiment. This study clearly shows that nutrient elution by water stratification can affect floral changes from diatom to flagellate dominance in summer in Omura Bay.
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