Geophysical Approach to Marine Coastal Ecology

This chapter gives basic information on Iriomote Island, the target area of Iriomote Island. Sections 1.1–1.3 describes geophysical aspect of Iriomote Island: the geographical characteristics, Sakiyamawan–Amitoriwan Nature Conservation Area, and the meteorology, respectively. Sections 1.4–1.6 describe biological aspects of Iriomote Island: Corals, Tropical sea grass Enhalus acoroides, Coenobita brevimanus, respectively.

Observations introduced into this book include current and wave observations mainly using WavesADCP, meteorological observation using an anemometer, and water quality observation using a portable multi-item water quality meter. This chapter presents a description of these observational methods.

This chapter presents a description of models used for simulations performed in this book, including the Coastal ocean Current Model with a multi-sigma coordinate system (CCM) developed by the author and a colleague for computation of seawater flow, a wave model SWAN developed at Delft University of Technology, and the Lagrangian particle tracking model.

The physical properties of coastal currents in semi-closed bays (e.g., Sakiyama and Amitori bays) in the northwestern part of Iriomote Island are discussed in this chapter, based on the results of theoretical, observational, and numerical investigations. According to the tidal flow dynamics in a single channel, these bays in this region are categorized as “deep bays,” in which little difference in phase and amplitude of the tidal levels exists from the open ocean. Ocean waters therefore flow in a bay in the incoming tide. By contrast, bay waters flow out to the open ocean in the outgoing tide. In Sakiyama Bay with its shallower waters, surface seawater exchange between the bay and ocean is active under near-calm conditions, leading to an environment in which the river water mass near the surface layer is more likely to flow out to the open ocean. By contrast, in Amitori Bay with the deeper waters, near-surface river water is stagnant under near-calm conditions. It selectively distributes to the eastern side of the bay because of the Earth’s rotation. The theory of tidal flow dynamics in a single channel suggests that differences of coastal currents between Sakiyama and Funauki bays are mainly attributable to the difference of the aspect ratio, the length to depth ratio, between the two bays. Such a difference in physical properties in the coastal currents is expected to affect the coastal ocean ecosystem distribution in this region.

This chapter presents investigation of the physical properties of tidal flows in two semi-closed bays connected with a channel based on theoretical, observational and numerical frameworks. Shirahama Bay with shallower water and Funauki Bay with deeper water in the northwestern part of Iriomote Island are mutually connected by a channel. Tidal flows in both bays are expected to be much more complex than those of Sakiyama and Amitori bays, discussed in Chap. 4. According to the tidal flow dynamics in Chap. 4, the seawater exchange in Shirahama Bay is more active because of its shallower water floor, whereas the seawater exchange in Funauki Bay is more stagnant because of its deeper water floor. Consequently, the low-salinity water originating from a river is likely to be retarded in Funauki Bay under near-calm conditions. In the case of a smaller aspect ratio (shallower and longer) of the connecting channel, the seawater exchange among the bays becomes inactive. In the case of the moderate aspect ratio of the connecting channel, which is comparable with those of both bays, the seawater exchange among the bays reaches a maximum. For a larger aspect ratio (deeper or shorter) of the connecting channel, the seawater exchange will stagnate not only between the bays, but also between the bays and the open ocean. The aspect ratio of the connecting channel, which strongly affects the tidal currents in both bays, is expected to strongly influence on the dispersions of eggs and seeds in the coastal ocean ecosystems in this region.

This chapter focuses on a technique for directly measuring suspended sediment or directly collecting coastal sediment for analysis. Simple and low-cost techniques employing sediment traps using a polyvinyl chloride pipe to capture suspended sediment and a handheld analyzer for X-ray fluorescent analysis and a digital scanner for image analysis were employed for sediment measurement and analysis. In laboratory experiments, the sediment catch performance of a cylindrical sediment trap was evaluated according to differences in the intake and outlet sizes. The characteristics of the measurement principle of the X-ray fluorescence analysis were described. Furthermore, a technique for digital image analysis that can estimate sediment diameter was constructed. In addition, color measurements using the CIE L* a* b* color space system ascertained sediment color characteristics. Next, these measurement and analysis techniques were applied to sediment monitoring in the Sakiyama and Amitori bays. Fine sediment was captured in the sediment traps. The element content of the coastal sediment from multiple points within the bays and their spatial distributions obtained through X-ray fluorescence analysis were ascertained. The color, diameter, and chemical composition of the sediments were classified using clustering analysis, and the distribution of the cluster within the bays matched the topographical features.

The main objective of this study is to demonstrate the possibility of underwater three-dimensional measurement (underwater SfM) in a coral reef area by structure from motion (SfM). The possibility of underwater SfM using a small submarine (small remotely operated vehicle, sROV) was also investigated. After we conducted underwater SfM and conventional measurements simultaneously for dendritic coral, massive coral, and seaweed Enhalus acoroides in a coral reef area in Amitori Bay of Iriomote Island, we compared and examined the results. Underwater SfM allowed us to measure the heights, major and minor axes, and branch thicknesses of dendritic coral with the accuracy of a maximum and minimum error of 4.5 cm and 0.1 cm, respectively. Consequently, it was verified to be useful for monitoring coral preservation. However, three-dimensional measurements of Enhalus acoroides by SfM were not satisfactory because of swaying of leaves by waves or currents during the taking of sequential photographs. Investigation by sROV provided a clear three-dimensional model, but some technological problems were also revealed.

The relation between coral distribution and physical characteristics was investigated for Amitori Bay, Iriomote Island, Japan. Field observations were conducted to obtain data for the coral distribution, sea temperature, sea salinity, wind speed, and river flow rate. Then, the observed data were used in ocean and wave model numerical simulations and soil particle tracking analyses to ascertain the spatial and temporal distributions of wave height and the numbers of soil particles. The main results of this study indicate that the wave height and the number of soil particles are significantly correlated with the coral distribution. Higher wave heights produce greater coverage of tabular coral and lower coverage of branching coral. A greater number of soil particles are related to less coverage of tabular coral. In contrast, the number of soil particles is not correlated with branching coral coverage. Averages of the diversity index of the coral types at the mouth and inner parts of the bay are lower than the average of the whole region, but the average of diversity index at the intermediate part of the bay with the intermediate physical disturbances is higher than the average, which supports the intermediate disturbance hypothesis.

Amitori Bay, located in northwestern Iriomote Island, is characterized by its varied physical environments such as geographical features, wave height, and current in spite of its small size. It also exhibits a diverse distribution of coral reefs in response. Physical data acquired through numerical analysis, although including errors from actual measurements, provide much information by being interpolated spatially and are useful for the understanding of phenomena. However, the spatial distribution of corals is difficult to estimate using an ecosystem model because the coral ecology has numerous and important unknown characteristics. We have proposed a new technique for estimation of the horizontal distribution of coral coverage based on the relevance between coral coverage and physical data. We have conducted estimation of horizontal distribution of coral in Amitori Bay in this study. A technique using a piecewise linear function with artificial intelligence as the analytic method has yielded excellent results compared to those of multiple regression analysis.

Enhalus acoroides, tropical sea grass also known as tape grass, is distributed throughout the Yaeyama Islands. It is listed as a vulnerable species (VU) on the Red List of the Ministry of Environment of Japan. Because this species is known to have poor seed dispersal characteristics, necessary conditions of its distribution must be elucidated to support its preservation. We have compared the present community distribution of E. acoroides with the horizontal distribution of numerically reproduced wave height and bottom velocity. Moreover, we have derived the conditions of water depth, waves, and bottom velocity required for the formation of E. acoroides distribution. The periphery of distribution of communities was determined using external forces during a high-wave period such as a typhoon period or a winter monsoon period.

This study was conducted to assess numerical simulation methods used to examine the initial dynamic state of coral bundle that has not been studied because of a shortage of observational data. For that purpose, we conducted field observations of spawning of Acroporidae in Amitori Bay, Iriomote Island, Japan. Results revealed spawning sites of Acropora digitifera, of which mass spawning were observed, the number of the spawning egg–sperm bundle and the rise rate of the bundle. After we conducted numerical simulations of the initial dynamic state of Acropora digitifera bundles based on observational data, we reproduced initial dynamic state characteristics such as the bundle rise and dispersion.

This study used numerical analysis to evaluate Enhalus acoroides seed and fruit dispersal in the northwestern sea area of Iriomote Island, Japan. Input data for numerical simulations were provided based on results of an ecological investigation of Enhalus acoroides and field observations of physical phenomena. Reproductions of coastal currents in the northwestern sea area of Iriomote Island were conducted using an ocean current model: CCM. Enhalus acoroides seed and fruit dispersal characteristics were examined using particle tracking analysis.

Recruitment of new individuals is essential for community sustainability and resilience. Spatial variation in coral recruitment was quantified for nine sites in Amitori Bay of Iriomote Island, Okinawa Prefecture, Japan, and compared with adult abundance. In addition, potential source sites for recruits, larval trajectories from source to sink sites, and travel distances of larvae were estimated by numerical modeling using real biological and physical data at the main spawning periods. As a result, acroporid recruitment was dominant in Amitori Bay and was positively correlated with adult abundance. Numerical modeling demonstrated a potential that recruits in Amitori Bay could be originated from nearby areas, and that most of recruits traveled less than 400 m with the maximum travel distance of less than 5.5 km. Therefore, estimated dispersal distance of larvae could be relatively short as the mouth of Amitori Bay is about 2 km across and the length of the bay is 4 km. These suggested that the acroporid community in Amitori Bay could be maintained by recruits from within the bay and from immediately surrounding areas.

Understanding pelagic larval dynamics is fundamentally necessary for biological conservation in marine biology. We analyzed larval transportation and recruitment of the land hermit crab Coenobita brevimanus during July 2010 in Amitori Bay on Iriomote Island by performing particle tracking analysis based on field observations of larval releases and weather conditions. Larval release was observed for several days before and after the new moon between July and September. It coincided well with the nighttime high tide. During July 10–14, 2010, it was estimated that 39 females released 780,000 zoeae. An average of about 10% of the larvae remained in the area near the release site. Many larvae were transported outside the bay rather than returning to the population. Therefore, the population of C. brevimanus in Amitori, which consists of many large crabs, is regarded as important as a source of recruits for adjacent habitats.