Algal Resources (2013) 6：1-13 Distributions of Zosteraceae species along the coasts of Aomori Prefecture, locating at the northernmost of Honshu, Japan Shinji KIRIHARA1*, Naoto KON２, Daisuke FUJITA３ and Masahiro NOTOYA４ Abstract : Distribution and standing crops of Zosteraceae species were surveyed along the coasts of Aomori Prefecture, locating at the northernmost of Honshu, Japan, from 1999 to 2002. Sampling were made at depths of 2.5 m (1,079 points), 5 m (1,181 points) and 10 m (1,183 points) on transects set at about 500 m intervals along the whole coasts divided in four areas according to oceanographic and topographic characteristics: the Japan Sea coastal area affected by Tsushima (warm) Current, the Pacific coastal area affected by Oyashio (cold) Current, coastal areas facing to the Tsugaru Strait connecting the two seas, and coastal areas of Mutsu Bay embayed by Tsugaru and Shimokita Peninsula. Distribution patterns of Zosteraceae species can be summarized as below. Phyllospadix iwatensis Makino: common on bedrocks along the Tsugaru Straight and Pacific coast but absent in Mutsu Bay; Zostera marina Linnaeus: common on soft bottoms of Mutsu Bay and Tsugaru Straight areas but absent in the Pacific coast; Z. caespitosa Miki: common on soft bottoms of Tsugaru Strait and Mutsu Bay; Z. caulescens Miki: restricted to a depth of 10 m in Tsugaru Strait coast. In Mutsu Bay, distributions of Z. marina and Z. caespitosa rarely overlapped and the latter species showed a tendency to grow in deeper depths. Sediment analyses of their habitats revealed that Z. caespitosa were likely to grow in the habitat with thinner sediment layer and coarser particle than Z. marina. Keywords : Zosteraceae, Zostera marina, Zostera caespitosa, Zostera caulescens, Phyllospadix iwatensis, distribution of seagrass, Aomori Prefecture Introduction Center of Japan 1994). Aomori Prefecture is located at the northernmost of Honshu Island Seagrass meadows are important for coastal and has the second largest seagrass meadows in environment and fisheries because they diver- total areas (7,271 ha) in Japan, following 17,071 sify both habitats and living organisms in ha of those in Hokkaido (Environment Agency coastal waters, improve water quality and pro- and Marine Parks Center of Japan 1994). mote sedimentation (Hemminga and Duarte The coastal area of Aomori Prefecture is 2000). In Japan, Zosteraceae species are major geographically divided into four sub-areas, that components of the seagrass meadows (Miki is, Japan Sea, Pacific Ocean, Tsugaru Strait and 1933) and the total area of the domestic Mutsu Bay areas. These areas are affected seagrass meadows was estimated to be 49,464 differently by both Tsushima (Tsugaru) warm ha (Environment Agency and Marine Parks and Oyashio cold Currents and show different 1 Fisheries Bureau, Aomori Prefectural Government, Aomori, Aomori 030-8570, Japan 2 MAC Co. Ltd., Kugurizaka, Aomori, Aomori 038-3212, Japan 3 Tokyo University of Marine Science and Technology, Konan, Minato-ku, Tokyo 108-8477, Japan 4 Notoya Research Institute of Applied Phycology, Taitoo-ku, Tokyo 108-8477, Japan ＊Correspondence author : e-mail : [email protected] 1 Shinji KIRIHARA,NaotoKON, Daisuke FUJITA and Masahiro NOTOYA Fig. 1. Map of Aomori Prefecture showing location of major survey points and area di- vision. oceanographic characteristics. In addition, topo- Materials and methods graphic characteristics are also quite different among them, e.g., though Japan Sea and Pacif- The biomass survey of Zosteraceae species ic Ocean area are directly facing to open oceans, was conducted from 1999 to 2002. In the sur- Mutsu Bay area is a semi-closed area which vey, 1,183 transects in total were set at about 500 is deeply embayed between Shimokita and m intervals along the whole coasts of Aomori Tsugaru Peninsula. Therefore, the distribution Prefecture except for port areas (Fig. 1). On each of Zosteraceae species along the coasts of the transect, sampling and observation were made Aomori Prefecture is interesting from the bio- at depths of 2.5 m (1,079 points), 5 m (1,181 geographical point of view. points) and 10 m (1,183 points) and totally 3,443 Furthermore, the seagrass meadows are used points were observed by SCUBA diving (Table as important fishery grounds, as well as spaw- 1). On transects set along a long sandy beach ning, nursery and feeding grounds of fish and which is approximately 50 km long in the shellfish (e.g. sea cucumbers, flounders) in Ao- middle of the Pacific coast area, sampling could mori Prefecture. However, information of the not be conducted at a depth of 2.5 m because seagrass meadows in Aomori Prefecture has of adverse physical conditions of high waves. been limited to parts of nation-wide floristic At each observation point, quadrates of 25m2 studies (Miki 1933; Tanaka et al. 1962; Omori (5 × 5 m) were placed and substratum charac- 1993, 2000) and a local biomass survey (Notoya teristics was categorized in the following five and Aruga 1992). Therefore, the authors con- categories; bedrock, boulder (diameter ca. 256 ducted a detailed survey on the distribution of mm ＜), gravel (2 - 256 mm), sand (0.1 - 2 mm) seagrass along the whole coasts of Aomori and mud (0.1 mm ＞). At soft substrata (sand or Prefecture from 1999 to 2002. In the present mud), sediments were sampled within the paper, we report the horizontal and vertical quadrates after thickness of sediments (up to 1 distribution of Zosteraceae species along the m in thickness) were measured using a gradu- coasts of Aomori Prefecture and discuss the ated bar. The sampled sediments were dried environmental factors affecting these distribu- and sieved to measure the size distribution of tional patterns. particles. Median particle diameter (φ50) of these samples were read from cumulative frequency curves about seven particle sizes of Wentworth 2 Distributions of Zosteraceae species along the coasts of Aomori Prefecture, locating at the northernmost of Honshu, Japan Table 1. Number of survey points in each district and depth (1922)'s class terms; granules (particle diameter standing crop. The frequency in occurrence is 2mm＜), very coarse sands (1 - 2 mm), coarse the ratio of the number of observation points sands (0.5 - 1 mm), medium sands (0.25 - 0.5 where the species appears to the total number mm), fine sands (0.125 - 0.25 mm), very fine of points. Standing crop of each species was sands (0.63 - 0.125 mm) and silts (0.063 mm ＞). measured in a wet weight of samples after they Asmallerquadrate(50× 50 cm = 0.25 m2)was were brought back to the laboratory at Aomori placed on a patch of each Zosteraceae species Prefectural Fisheries Research Center. in the large quadrate (25 m2)tosampleand Pentad average seawater temperatures from estimate standing crops. Distribution of each 1985 to 2002 on the nine sites (Fukaura, Aji- species along the coast of Aomori Prefecture gasawa, Mimmaya, Hiranai, Sai, Kazamaura, was described as frequency in occurrence and Rokkasho, Hachinohe and Hashikami) of Ao- 3 Shinji KIRIHARA,NaotoKON, Daisuke FUJITA and Masahiro NOTOYA mori Prefectural coasts were obtained from the data of“UODAS”report (Aomori Prefectural Fisheries Research Center 2004). Results Seawater temperature Seasonal changes in pentad average seawater temperature at the nine sites of Aomori Pre- fectural coasts are shown in Fig. 2. The aver- aged seawater temperatures of sites in the Ja- pan Sea area (Fukaura, Ajigasawa), at the northern coast of Tsugaru Peninsula (Mim- maya), at the coast of Mutsu Bay (Hiranai), at the northern coast of Shimokita Peninsula (Sai, Kazamaura)andatthecoastofthePacific Fig. 2. Seasonal changes of average sea water tem- Ocean (Rokkasho, Hachinohe, Hashikami) were perature at nine coastal sites in Aomori Pre- in the range of 6.6 - 25.5 ℃,7.7- 23.4 ℃,7.2- fecture (Aomori Prefectural Fisheries Research Center 2004). See Fig. 1 for alphabets in 23.7 ℃,6.6- 23.3 ℃ and 5.3 - 21.1 ℃, respectively. parentheses showing locations. There was a tendency that seawater tempera- ture was higher at the sites in the western ar- ea of Aomori Prefecture. substrata observed between Cape Shiriya and Cape Nakayama（‘v - w’）and near Hashika- Substratum characteristics mi（‘y-z’）. Distribution of five types of substrata on the Median particle diameters of sediments sam- whole coast of Aomori Prefecture is shown in pled from 2,009 points (58.4％ to total points) Fig. 3. dominated by soft bottom (sand or mud) were Along the Japan Sea coast, bedrock was showninTable2. dominant in the southern area（area‘b’）and The mean median particle diameter of sedi- northern area（‘f’）with the exception of a long ments averaged on five coasts (Japan Sea, sandy beach (about 40 km) in the middle area westernpartofthecoastfacingtoTsugaru （‘c - e’）.Intheareas‘g-i’along the coast Strait, Mutsu Bay, eastern part of the coast between Cape Tappi and Cape Myojin facing facing to Tsugaru Strait and the Pacific Ocean) to the Tsugaru Strait, coastal substrata were were in the range of 0.36 - 0.44 mm, 0.38 - 0.64 mainly composed of bedrocks or boulders with mm, 0.29 - 0.52 mm, 0.33 - 0.54 mm and 0.30 - the exception of the soft bottoms in Minmaya 0.32 mm, respectively. Bay（Fig. 3,‘g - h’）.AlongthecoastofMut- su Bay（‘i-q’）, sand, mud or gravel was Distributions and abundance of Zosteraceae dominant except for the area ‘l’along the species Natsudomari Peninsula coast where boulder In the present study, four Zosteraceae spe- was dominant. Along the coast of Ashizaki Bay cies, Zostera marina Linnaeus, Z. caespitosa （‘o, p’）, muddy sea bottom was prominently Miki, Z. caulescens Miki and Phyllospadix iwat- dominant. Along the coast of Shimokita Penin- ensis Makino were recorded along the coast of sula between Cape Kai and Cape Shiriya fac- Aomori Prefecture.