Effects of environmental factors on Sparganium emersum and Sparganium erectum colonization in two drainage ditches with different maintenance

In the Niheishimizu and Ooshimizu sections of the town of Misato in the Akita Prefecture, Northern Japan, there are many abundant spring water areas. Sparganium (Sparganium emersum and Sparganium erectum) species are widely distributed in the irrigation water that fed by spring water. The irrigation waters were divided the natural type ditch and the maintained ditch that connect with nearby natural ditch to promote environmentally friendly agriculture. This study was conducted in both sections to support the maintenance of the irrigation water fed by the abundant spring water. A vegetation survey was conducted in September of 2005. The survey collected data on the amount of vegetation cover and the stem lengths of the plant species found in selected locations of the study area. The water depths and the flow velocities were also measured in these locations. As for the growth situation of S. emersum and S. erectum, the submerged form of S. emersum was found in water approximately 15 cm deep with a surface flow velocity of approximately 7 cm/s. This species was characterised by a relatively fast flow and relatively shallow water. The emergent and submerged growth forms of S. emersum were found in waters having flow velocities faster than those associated with S. erectum. The emergent form of S. emersum grew in relatively deep water. S. emersum is more capable of adjusting to the conditions of stream habitats than S. erectum.


INTRODUCTION
has established the "Ministry of Agriculture, Forestry and Fisheries (MAFF) biodiversity strategy" to promote approaches to agriculture that recognise the value of biodiversity, reduce damage to the populations of local birds and other animals and protect agricultural resources.The MAFF is promoting types of environmentally friendly agriculture that can coexist with many living organisms.It is necessary to maintain and restore rice fields, ditch and the habitats of wild flora and fauna.
Spring water areas occur in some parts of the alluvial fans located on the plains in the northern area of the Senboku District in the Yokote basin, Akita Prefecture, Northern Japan.The spring water is primarily used to supply an irrigation water.Species included on the Red List of the [2] and [3] are found in the area, including Hippuris vulgaris, Sparganium erectum, Sparganium japonicum, Sparganium simplex, and Pungitius pungitius [4].In particular, Sparganium (S. emersum and S. erectum) has a very extensive distribution in the ditch of Niheishimizu and Ohshimizu sections of the town of Misato.Sparganium grows to water area where spring water is abundant, and these breeding seasons are August and September [5].We consider that Sparganium is suitable for growth with the above-mentioned ditch.However, recent years, the habitat of Sparganium (i.e., S. erectum, S. emersum and S. emetsum) shows the tendency to decrease according to the influence of the development of the river maintenance [6,7].In addition, basic information on the life history and habitat characteristics of Sparganium is lacking.Moreover, few studies have investigated the differences in distribution between Sparganium species (i.e., S. emersum and S. erectum) found in the same aquatic areas or in neighbouring areas.
In this study, we examined the differences in growth situation between different species Sparganium (S. emersum and S. erectum) by comparing the state of the vegetation and the growth environment (water depth, flow velocity and the maintenance) of these species in the abundant ditch of spring water.

Study Site
The study sites were located in a spring water area in the Niheishimizu and Ooshimizu sections of the town of Misato, Akita prefecture, Northern Japan (Figure 1).
We investigated areas located upstream (Site 1) and downstream (Site 2) of the weir in the Ooshimizu, upstream (Site 3) and downstream (Site 4) of the weir in the Niheishimizu.
The ditch in the Niheishimizu section is maintained by protecting the banks with stones placed in the channel and was originally created and maintained by a farmland consolidation project in 2003.The bottom of the ditch is lined with sling stone, and Sparganium plants which were grown in the neighbourhood have been transplanted to the bottom of the ditch.The ditch in the Ooshimizu section has been left in nearby natural form.For the purposes of this study, Niheishimizu was referred as to be a maintained ditch, and Ooshimizu was referred as to be a natural type ditch.

Sampling and Identification Methods
The investigation was conducted during September of 2005.The vegetation was surveyed using a quadrant frame (50 × 50 cm 2 ) in each section.The numbers of quadrant frames were 15, 23 in upstream (Site 1) and downstream (Site 2) of the weir in the Ooshimizu, and were 18, 26 in upstream (Site 3) and downstream (Site 4) of the weir in the Niheishimizu.The amount of vegetation cover for each plant species summed the cover of a quadrant for these.The stem length was measured in same quadrant in which vegetation cover was recorded.The stem length included the above-ground portion of the plant in the central of quadrant frame and the submerged portion of the plants in the stream area was included in this measurement.The flow velocity (surface and bottom), the water depth and the vegetation cover were measured at the same time.The flow velocity was measured with a KENEK VP-3000 three-dimensional electromagnetic current meter.The surface measured approximately 3cm under water surface and the bottom measured approximately 1 cm above the stream bed.The following diagnostic characteristics were used to identify Sparganium species: S. erectum has a divided scape and 3 or more branches, and the upper part of the seed extends above the dome; S. emersum has an undivided scape and 4 or more staminate heads, and the pistillate heads above the second are axillary.Moreover, as for S.erectum, the dorsal crest of the leaf develops, and the section is triangular.The leaf is 7 -20 mm in width.As for S. emersum, the emerged leaf is 5 -16 mm in width, dorsal crest develops a little.The submerged leaf is 6 -9 mm in width, the dorsal crest is not remarkable [5].

Vegetation Coverage
Upstream of the weir in the maintained ditch, S. emersum and S. erectum covered areas of 2.58 m 2 and 0.94 m 2 , respectively.Their coverage downstream of the weir were 1.83 m 2 and 3.7 m 2 , respectively, and Typha latifolia covered 0.35 m 2 downstream of the weir.In the natural type ditch, S. emersum covered 2.64 m 2 upstream of the weir, and S. emersum and S. erectum covered 1.46 m 2 and 1.12 m 2 , respectively, downstream of the weir (Table 1).

Flow Velocity and Water Depth
The flow velocity was the highest upstream of the weir in the natural type ditch.The flow velocity was successively lower downstream of the weir in the natural type ditch, downstream of the weir in the maintained ditch and upstream of the weir in the maintained ditch.The water was the deepest upstream of the weir in the maintained ditch.The water depth was found to be successively reduced downstream of the weir in the maintained ditch, downstream of the weir in the natural type ditch and upstream of the weir in the natural type ditch.
The relative coverage of the emergent and submerged growth forms of the plants differed among the sampling sites.The emergent form showed a coverage of 100% upstream of the weir in the maintained ditch, and the submerged form showed a coverage of 100% upstream of the weir in the natural type ditch.Overall, the relative abundance of the submerged form increased with increasing flow velocity and decreasing water depth (Figure 2).

The Relative amounts of Vegetation
Cover according to Species,

Stem-Length Class and Growth Form
The emergent form of S. emersum was found both upstream and downstream of the weir in the maintained ditch.The relative amount of vegetation cover represented by the short stem lengths was higher in these locations than in the location downstream of the weir in the natural type ditch.The submerged growth form was found upstream of the weir in the natural type ditch, and both growth forms were found downstream of the weir in the natural type ditch.In the natural type ditch, the relative coverage of the shortest stem-length class was higher upstream of the weir than downstream of the weir.
The emergent form of S. erectum upstream of the weir in the maintained ditch showed the highest relative amount of vegetation cover in the stem-length class of 1.5 -2.0 m.Both growth forms of this species were found downstream of the weir in the maintained ditch and downstream of the weir in the natural type ditch.S. erectum was not found upstream of the weir in the natural type ditch (Figure 3).

The Growth Environment (Flow Velocity and Water Depth) of the Species and Growth Forms of Sparganium
The surface and bottom flow velocities were nearly equal in each case studied.The submerged form of S. emersum was found in the fastest-flowing, shallowest water.The emergent form of S. erectum was found in even slower and even deeper water.The emergent form of S. emersum was found in water having flow velocities similar to those associated with the submerged form of S. erectum and in water deeper than that associated with the other growth forms of both species (Figure 4).

The Growth Situation of Sparganium in Each Sampling Location
The emergent form of S. erectum was only found upstream of the weir in the maintained ditch.The relative vegetation cover was the highest for the stem-length class of 1.5 -2.0 m.Both growth forms of S. erectum were found downstream of the weir in the maintained ditch and downstream of the weir in the natural type ditch.S. erectum was not found upstream of the weir in the natural type ditch (Figure 3).
S. erectum grows in lotic and lentic environments.Many species of this genus grow as a submerged form in running water [7].However, the emergent of S. erectum occur commonly at the margins of low-and mediumenergy river systems across the northern temperature zone [8].S. erectum grows densely in gradually flowing waters that have a flow velocity of less than 5 cm/s, it is rarely found in rapidly flowing waters that have a velocity of 5 cm/s or more.Indeed, it has been reported to grow tall in a deep, gently flowing stream [9].
Upstream and downstream of the weir in the maintained ditch, an extremely gradual flow of less than 5 cm/s was recorded, and the water depth was 30 cm or greater.Downstream of the weir in the natural type ditch, the flow velocity exceeded 5 cm/s, and the water depth was (16.73 ± 0.34) cm, a relatively low value.We hypothesised that the emergent form of S. erectum grows by extending its stems and expanding its distribution in waters whose flow velocity is less than 5 cm/s and whose depth exceeds 30 cm.If the flow velocity exceeds 5 cm/s and the water depth is approximately 15 cm or greater, the plant must develop a submerged growth form.Upstream of the weir in the natural type ditch, only the submerged form of S. emersum was found.The relative vegetation cover was highest for the shortest stem-length class of 0 -0.5 m.
S. emersum is dominated in waters where a fast flow and disturbance are strong, and adapted by the submerged type as these influences strengthens in comparison to S. erectum [10].The submerged form of S. emersum grows in running waters.It grows densely at bottom flow velocities (measured approximately 1 cm above the  stream bed) of (3.9 ± 0.4) cm/s to (5.9 ± 2.4) cm/s [11].In this study, the stream bed flow velocity upstream of the weir in the natural type ditch was (5.3 ± 0.3) cm/s, and the water depth was (13.6 ± 0.15) cm (Figure 2).
We hypothesised that the submerged form of S. emersum is suitable for these environmental conditions because the plant controls its growth as the flow velocity increases and the depth decreases.Although the stems of S. emersum tend to be shorter than those of S. erectum, the short stem might enhance the capability to resist strong drag in fast flow.
As other study cases, S. erectum grows in waters where the flow is comparatively gradual and sand and silt accumulate [12].S. emersum grows well in a gradual flow, mud, and argillaceous soil [13], and it also grows in sandy, muddy sediment with constant water flow velocities and is found in eutrophic conditions [14].To clarify the factors affecting the growth of the two species, future research on Sparganium habitats should include investigations of the water quality and the soil substrate.

Figure 2 .
Figure 2. The rate of vegetation cover on each life form of aquatic macropyhtes (Total values), flow velocity and water depth in each investigation section.※The vertical bars indicate a standard error.N shows the number of measurement.

Figure 3 .
Figure 3.The relative amounts of vegetation cover of stem length class of life form on Sparganium (S. emersum, S. erectum) in investigation section (%).

Figure 4 .
Figure 4.The relationship flow velocity, water depth and stem length in habitat of Sparganium (S. emersum, S. erectum ).※The vertical and horizontal bars indicate a standard error.

Table 1 .
The vegetation cover areas (m 2 ) of aquatic botany founded in the investigation ground.