Open Access
Issue
Knowl. Manag. Aquat. Ecosyst.
Number 417, 2016
Article Number 12
Number of page(s) 5
DOI https://doi.org/10.1051/kmae/2015045
Published online 12 February 2016

© T. Komuro et al., published by EDP Sciences, 2016

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In shallow lakes, submerged aquatic vegetation (SAV) plays a vital role in maintaining a clear stable state (Scheffer et al., 2001). However, previous studies have shown that angiosperms are not as effective in maintaining a clear stable state as charophytes (Van den Berg et al., 1998; Nõges et al., 2005).

thumbnail Fig. 1

Location of seven sampling stations (Stations 1–7) in Lake Shinji. Sampling was conducted at Stations 1–7 in 2011, and Stations 2, 4, and 5 in 2013.

Lake Shinji, Japan, is an oligohaline lagoon with a salinity range of 1–10 psu (Figure 1). The lake area is 79.1 km2 and the basin area 1288.4 km2. The maximum depth is 6.4 m, while the average depth is 4.5 m. Hiratsuka et al. (2006) indicated that SAV (unknown species) exists up to 2–3 m depth. Additionally, Komuro and Yamamuro (2013) examined an airborne photograph taken in 1947 by the US Army, and showed that creeping type SAV covered the bottom of Lake Shinji up to 3 m depth. SAV was previously used as a fertilizer; however, it almost disappeared from Lake Shinji after the beginning of herbicide use for rice weed control in the mid-1950s (Yamamuro, 2012).

Akiyama and Nishigami (1959) collected Chara braunii C.C. Gmelin in 1958, and Kasaki (1964) collected C. braunii and Nitella hyalina (De Candolle) C. Agardh until the late 1960s. Both studies referred to samples that were collected from the shoreline of Lake Shinji; thus, it is possible that creeping type SAV at 2–3 m depth comprised other species than those reported in these two studies.

The amount of herbicides used in rice paddies began to decrease after the implementation of a law on the restricted use of herbicides in 2007. As a result, Potamogeton anguillanus Koidz. started to grow in the shallow parts of Lake Shinji (Yamamuro et al., 2014), and its canopy reached the water surface. However, transparency did not increase by the mass coverage of SAV (Yamamuro et al., 2014), which is in disagreement with the alternative stable state theory (Scheffer et al., 2001). A possible explanation could be that Lake Shinji was transparent by the 1960s, because SAV was mostly composed of charophytes that are considered more effective in increasing transparency (Blindow et al., 2002; Van Donk et al., 2002). The aim of this study was to investigate the composition of past SAV by collecting sediments and conducting seed analysis, in order to show that charophytes, and not P. anguillanus, were dominant when Lake Shinji was transparent before the loss of vegetation due to herbicides.

Sediments were collected from seven stations (Stations 1–7) of Lake Shinji at depths of more than 5 m, where 99% of the sediment is composed of silt and mud (Figure 1). The sedimentation rates of these seven stations were reported by Kanai et al. (1997); thus, it was possible to determine the portion of core sediment that accumulated prior to 1960. The sediment core collection was performed in August 2011 and 2013. In 2011, we sampled all seven stations, and in 2013, we sampled three stations (Stations 2, 4, and 5). A professional diver dived to each station, and gently placed an acrylic tube (8.5 cm diameter, 102 cm length) on the sediment surface in order to prevent any disturbance. The top of the tube was closed with a silicon lid, the tube was pulled out of the sediment surface, and then the bottom of the tube was closed with another silicon lid.

thumbnail Fig. 2

A representation of the sieve used for sediment sampling in Lake Shinji. Numbers in μm denote the mesh size of the sieve and those in cm show the diameter of the sieve. The top sieve (30-cm diameter) is made of stainless steel, while the remaining part is made of plankton net cloth (Komuro and Yamamuro, 2012).

The length of cores was approximately 70 cm. Since the sediment accumulation rate of the sampling station is 0.04–0.31 cm·y-1 (Kanai et al., 1997), we removed the top 2–15-cm sediment layer to exclude sediment that accumulated after 1960. We also discarded sediment from a depth of approximately 20 cm from the bottom layer, because the mud was tightly fixed by diagenesis and precluded sieving. To reduce the amount of sediment required to obtain seeds and oospores by seed analysis, we used a specially designed net sieve as described by Komuro and Yamamuro (2011) (Figure 2). To collect seeds of angiosperms and charophyte oospores, the sediment was sieved through 100-μm and 250-μm mesh sieves (Birks, 2001). The particles that remained on the sieves were transferred to the laboratory and immediately sieved through a 100-μm mesh stainless-steel sieve under running water. The particles that remained on the sieve were dried in an oven at 40 °C until constant weight was reached. The dried particles were sieved again through 100-μm and 250-μm mesh stainless steel sieves. Since charophyte oospores are usually 200–900 μm in length and 200–500 μm in width, we expected that most oospores would be retained on the 250-μm mesh sieve. The remaining particles were separately examined using a digital binocular microscope (Keyence, Osaka, Japan) in order to collect seeds and oospores.

Table 1

Dry weight (DW) of sediments (collected in 2011) on the 250- and 100-μm mesh sieves.

Because the length of sediments used for seed analysis in 2011 differed between the sampling stations, the dry weight (DW) of sediments on the 250-μm mesh, as well that on the 100-μm mesh also differed (Table 1). We did not measure the DW of sediments on the 100-μm mesh and 250-μm mesh that were used for seed analysis in 2013. We assumed that coarse particles predominated at Station 1, which is near the river mouth, while mud predominated at all other stations; thus, most sediments were lost during sieving.

Charophyte oospores were observed under a scanning electron microscope (SEM) as described by Sakayama et al. (2002) with some modifications. Before observation, oospores were cleaned by incubation in 10% Triton-X 100 at 60 °C for more than 12 h, subjected to acetolysis or sonicated for approximately 2 min, and transferred to centrifuge tubes with distilled water. Acetolysis was not performed in species that had fragile flanges on the surface of oospores. Then, oospores were dehydrated in an ethanol series (50% and 70%), mounted on brass or aluminum stubs with double-side adhesive tape, air-dried, sputter-coated with gold, and viewed under S-2150N and S-4800 scanning electron microscopes (Hitachi, Tokyo, Japan) at 10–20 kV. Oospore identification was performed as described by Kasaki (1964), Wood (1965), Imahori and Kasaki (1977), Casanova (2013), and Casanova and Karol (2014) and maintained at the University of Tokyo, Kashiwa, Japan.

Angiosperm seeds were not obtained in any of the sediments; however, charophyte oospores were identified in sediments sampled in both 2011 and 2013. The total number of oospores obtained in two years was 49 (Figure 3). Detailed information on the number of oospores at each station in 2011 and 2013 is presented in Table 2. Using SEM, we identified four different charophyte species, C. braunii, Chara corallina Willdenow, Chara fibrosa C. Agardh ex Bruzelius, and Chara sp. (Figure 4). We obtained 35 oospores of C. corallina, 11 of C. braunii, two of C. fibrosa, and one of Chara sp. (Table 2). Overall, C. corallina was the most abundant species, in terms of oospore number and the number of stations obtained from. To the best of our knowledge, this is the first record of C. corallina and C. fibrosa in Lake Shinji.

thumbnail Fig. 3

Number of Chara corallina, Chara braunii, Chara. fibrosa, and Chara sp. oospores collected from Lake Shinji in 2011 and 2013.

Table 2

Number of Chara corallina, Chara braunii, Chara fibrosa, and Chara sp. oospores collected from Lake Shinji (Stations 1–7) in 2011 and 2013.

The number of oospores per DW of sediment (>100-μm) varied from 0.05 to 1.09 among sampling stations (0.05 oospores·g-1 of DW sediment, 0.22 oospores·g-1 of DW sediment, 0.37 oospores·g-1 of DW sediment, 0.92 oospores·g-1 of DW sediment, 1.09 oospores·g-1 of DW sediment, 0.64 oospores·g-1 of DW sediment, and 0.64 oospores·g-1 of DW sediment at Stations 1, 2, 3, 4, 5, 6, and 7, respectively). These densities were much lower than those in previous studies on recent fossil oospores (i.e., Davidson et al., 2005; Rodrigo et al., 2010). For example, the number of oospores and gyrogonites in the sediments of Albufera de Valéncia lagoon (area 23 km2, mean depth 1–2 m) was more than 30 (Rodrigo et al., 2010). In Lake Shinji, creeping type SAV, possibly charophytes, covered the bottom up to 3 m depth (Komuro and Yamamuro, 2013). Since Lake Shinji is bigger than other studied lakes (i.e., Groby Pool, area 0.12 km2; Davidson et al., 2005), sediments at up to 3 m, composed of sand and organic materials, are removed by strong wave actions and transported to deeper lake basin depths, which may explain the fact that we collected sediment samples from depths exceeding 5 m. In such case, the density of oospores is diluted during transportation and sedimentation.

thumbnail Fig. 4

Scanning electron micrograph of (A) Chara braunii, (B) Chara corallina, (C) Chara fibrosa, and (D) Chara sp.

Before the 1950s, when herbicides were not used in rice paddies, the transparency of Lake Shinji was assumed to be 4 m (Komuro and Yamamuro, 2013) and, based on an airborne photograph, SAV was of creeping type. After 2007 that the use of herbicides decreased, P. anguillanus covered the shallows of Lake Shinji; however, transparency remained stable at approximately 1 m (Yamamuro et al., 2014). Our results showed that the sediments of Lake Shinji before 1960 included only charophytes, which is in accordance with our previous study (Komuro and Yamamuro, 2013).

The present study suggested that charophytes play an important role in maintaining the water transparency in the lake, and thus, invasive angiosperms did not play any role in terms of improving transparency. We found that C. corallina was the most abundant charophyte before the reduction in SAV due to herbicide use. Among calcified charophytes,C. corallina contains relatively high calcium content (Kawahata et al., 2013). Calcium can induce the clear water state by absorbing the phosphate in the water column (Hilt et al., 2006).

This study indicates that seed analysis is helpful in reconstructing the former flora, which has not been recorded in the literature. The flora before the use of herbicides differed from that which appeared after the reduction in herbicide use, probably because of the invasion of exotic species in the lake. Overall, these results show that the flora appearing after the decrease in human disturbance does not always revive the original flora. Thus, our study contributes important information that can be useful in conservation and management programs of aquatic ecosystems.

Acknowledgments

We would like to thank Mr. Yukitaka Koyama and Mr. Yuki Nakamura for their help during collecting sediments. We would also like to thank Ms. Yui Nakajima for her help during the preparation for field work, and Ms. Misato Shimizu and Ms. Aoi Shibata for their kind assistance with SEM observations. This work was supported by the Grants-in-Aid for Scientific Research (KAKENHI) (Nos. 24310053 and 24570100) and the River Works Technology Research and Development Program of the Japanese Ministry of Land, Infrastructure, Transport, and Tourism.

References

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Cite this article as: T. Komuro, H. Sakayamai, H. Kamiya, M. Yamamuro, 2016. Reconstruction of the charophyte community of Lake Shinji by oospore collection. Knowl. Manag. Aquat. Ecosyst., 417, 12.

All Tables

Table 1

Dry weight (DW) of sediments (collected in 2011) on the 250- and 100-μm mesh sieves.

Table 2

Number of Chara corallina, Chara braunii, Chara fibrosa, and Chara sp. oospores collected from Lake Shinji (Stations 1–7) in 2011 and 2013.

All Figures

thumbnail Fig. 1

Location of seven sampling stations (Stations 1–7) in Lake Shinji. Sampling was conducted at Stations 1–7 in 2011, and Stations 2, 4, and 5 in 2013.

In the text
thumbnail Fig. 2

A representation of the sieve used for sediment sampling in Lake Shinji. Numbers in μm denote the mesh size of the sieve and those in cm show the diameter of the sieve. The top sieve (30-cm diameter) is made of stainless steel, while the remaining part is made of plankton net cloth (Komuro and Yamamuro, 2012).

In the text
thumbnail Fig. 3

Number of Chara corallina, Chara braunii, Chara. fibrosa, and Chara sp. oospores collected from Lake Shinji in 2011 and 2013.

In the text
thumbnail Fig. 4

Scanning electron micrograph of (A) Chara braunii, (B) Chara corallina, (C) Chara fibrosa, and (D) Chara sp.

In the text

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