Hidenobu Kunii

Seasonal growth and profile structure development of Elodea nuttallii (Planch.) St. John in pond Ojaga-ike, Japan

Abstract Phenological and quantitative observations on Elodea nuttallii (Planch.) St. John, an exotic aquatic plant in Japan, were made in a shallow pond throughout 1979. Shoot elongation began in spring (late March) when the bottom water temperature became higher than about 10°C. Elongation ceased when the shoot apices reached the pond surface and vigorous branching then occurred. The community formed a dense canopy, with 40–65% of the shoot biomass in the topmost 30-cm water layer during the growing season. Maximum plant biomass (712 g dry wt. m −2 ) was attained in late July, while the peak root biomass occurred around June, coincident with peak flowering. The anchoring roots and stems eventually died, and after September, the population existed as a floating mat of non-anchored leafy short shoots and decaying old branch stems. This mat sank suddenly to the bottom in December, when water temperatures dropped below approximately 10°C, and overwintered there. The ecological significance of the perennial growth habit and the formation of a floating mat is discussed in terms of the adventive spread of this plant, and an estimation of annual net production and P/B quotient is also made.

Life cycle and growth ofPotamogeton crispus L. in a shallow pond, ojaga-ike

The life cycle and growth ofPotamogeton crispus L. were studied in a shallow pond, Ojaga-ike. With respect to the shoot elongation and seed and turion formations, the life cycle of this plant in the pond could be divided into following five stages: germination, inactive growth, active growth, reproductive and dormant stages. It was suggested that the plant showed these successive stages depending mainly upon water temperature. The turions germinated on the bottom in autumn when the water temperature fell below ca. 20 C. The plant showed hardly any growth during winter (December—early March) when the temperature was below 10 C. In the spring when the bottom water temperature rose to above 10 C (mid-March), the plant started to grow again and the shoot elongated rapidly at the rate of 4.2 cm day−1 until the shoot apex reached the pond surface in late April. Both the increment of node number and the internodal elongation were associated with this rapid shoot growth. On 10 May (last sampling date), the mean values of shoot length, internodal length and the number of nodes estimated for 10 predominant plants were 238.2±5.6 cm, 7.1±0.8 cm and 34.9±4.0 cm, respectively. The turion formation and flowering occurred during the period from mid-April to mid-May when the surface water temperature ranged 19 and 22 C. The dry weight of a plant reached the maximum mean value of 1180 mg on 10 May. At its peak biomass, an individual plant produced 1–10 turions (5.5 on average) of which the mean individual turion dry weight was 53.2 mg. The turion dry weight accounted for ca. 42% of the total plant biomass m−2 at that time.

Seasonal and long-term changes in surface cover of aquatic plants in a shallow pond, Ojaga-ike, Chiba, Japan

Seasonal changes in surface cover and long-term floral changes in aquatic plants of a shallow eutrophic pond, Ojaga-ike, as well as the phenology of each species, were studied. Out of ten floating-leaved and submerged plants, the following five species showed a change in surface cover during 1978: Potamogeton crispus and Elodea nuttallii occupied 53.5% of the total pond surface in May, Trapa natans and Nelumbo nucifera 79.1% in July, N. nucifera and Hydrilla verticillata 62.0% in September and N. nucifera 41.2% in November. In addition to dramatic seasonal changes in surface cover, long-term floral changes occurred in this pond from 1971 onwards. While Nymphoides indica, Myriophyllum spicatum and H. verticillata decreased in number and in distribution ranges between 1971 and 1978, two newly established plants, N. nucifera and E. nuttallii, have now become dominant. The alteration in dominance may be due to water level fluctuations and interspecific competition for light. Seasonal changes in water quality during the study period are also summarized.

Continuous growth and clump maintenance of Potamogeton crispus L. in Narutoh River, Japan

Abstract To account for the continuous growth and clump maintenance of Potamogeton crispus L. in a lotic habitat, phenological and quantitative observations were made in a small river in Japan from April 1981 to August 1982. Favourable water temperatures (8.7–24.7°C) for shoot growth prevailed throughout the study period and, thus, the adventitiously rooting shoots grew successively from buried stems and rhizomes all the year round. Total stem length (including above- and underground stems) reached a maximum of 4690 m m −2 in April 1982. Flowering and turion formation occurred almost simultaneously from late May until late October when water temperatures were higher than 20°C and the photoperiod was longer than 12 h. Though the plant produced numerous turions (> 9600 turions m −2 year −1 ), they were small (5–15 mg dry wt. turion −1 ) and assumed to be less important than underground stems for clump maintenance; while the stems were firmly rooted in bottom sand and protected by sediment from being swept away, most of turions were washed downstream immediately after they detached from the parent shoots. Among the total number of turions collected, only 2.7% were those which remained and germinated in the clump. Distinctive features of this plant in lotic and lentic habitats are also presented.

Aquatic macrophyte composition in relation to environmental factors of irrigation ponds around Lake Shinji, Shimane, Japan

A comparative study of aquatic vegetation of 149 irrigation ponds in the surrounding area of Lake Shinji, Shimane Prefecture, Japan, was done to document the present status of aquatic flora and water chemistry. In addition, influence of 17 environmental factors on the between-pond variation in aquatic macrophyte composition was assessed by using detrended correspondence analysis (DCA). A total of 38 taxa were recorded, and the most frequent taxon was Trapa spp. (>50%). Number of taxa per pond ranged from 1 to 14 and mean number was 4.1. Floristic composition was primarily related to variation of the factors such as conductivity, Mg, Ca, Na, alkalinity, altitude, pH and depth. In contrast, no significant correlations were found with total P, PO4-P, transparency, COD, ignition loss of bottom soil, Mn and Fe. Of the 33 significant species associations between 19 species with more than 5% frequency occurrence, 28 were positive and the remaining 5 were negative. All of these negative associations involved Trapa spp., and not only water chemistry but some other factors like competition and bottom soil preference were assumed to influence species associations.

Characteristics of the winter growth of detached Elodea nuttallii (Planch.) St. John in Japan.

Abstract The winter growth of an introduced aquatic plant, Elodea nuttallii (Planch.) St. John was observed in Japan with regard to the water temperature. It was found that the plant could grow during winter when the daily mean water temperature was higher than 4°C. The lateral shoots and roots in parts which detached spontaneously from the apical portion of the mother shoots grew faster than those in undetached apical portions.

Temporal and spatial variation in the macrophyte distribution in coastal lagoon Lake Nakaumi and its neighboring waters

During the period May–December 1996, macrophyte distribution and its abundance were investigated for 24 sites in coastal lagoon Lake Nakaumi and its neighboring waters, southwestern Honshu, Japan. More than 21 taxa, including two aquatic angiosperms, were found and the taxa with high frequency of occurrence were Enteromorpha spp. and Sargassum thunbergii. Ruppia maritima, one of the endangered aquatic macrophytes in Japan, was also found making dense meadows in Honjou, an area planned to be reclaimed. Four distinct distributional regions were recognized in the lake by principal component analysis (PCA) using the seasonal maximum cover of each species, and a significant relation was found between PCA axis 1 and electric conductivity. A long-term change in floristic composition is also discussed based on the present results and literature.

Annual net production and life span of floating leaves in Nymphaea tetragona Georgi: a comparison with other floating-leaved macrophytes

Quantitative observation on the seasonal changes in leaf life span, leaf area and biomass of a floating-leaved aquatic plant, Nymphaea tetragona Georgi, were carried out for a year in a small irrigation pond, Japan, by using a non-destructive leaf-marking method. Annual mean life span and turnover rate of floating leaves in number were estimated at 31.0 days and 6.7, respectively. Seasonal maximum leaf area index was 1.50–2.04 m2 m−2, and seasonal maximum biomass and annual net production of aboveground parts (leaf-blades and petioles) were 133.0–173.2 g dry wt. m−2 and 515.0–559.0 g dry wt. m−2 y−1, respectively. The P/Bmax ratio of aboveground parts was thus as high as 3.2–3.9. Nearly 60% of the summer biomass was accounted for by the underground parts (roots and rhizomes), and P/Bmax ratio of whole plant was arbitrarily estimated as 1.8–2.0 provided that turnover rate of underground parts was 1.0. These values are generally similar to those of other floating-leaved plants and further confirm the rapid turnover rate and high productivity of floating leaves.

Seasonal growth and biomass ofTrapa japonica Flerov in Ojaga-Ike Pond, Chiba, Japan

In order to determine the seasonal growth and biomass ofTrapa japonica Flerov, field observations were carried out at Ojaga-ike Pond, Chiba, Japan, during 1979 and 1980. In spring, the plant showed exponential growth (c. 0.080 g g−1 day−1) and shoot elongation was as rapid as 10 cm day−1. The plant attained its maximum biomass (380.5±35.1 g m−2) in late August, and about 50% of this was concentrated in the topmost 30-cm stratum (645.7±33.1 g m−3); maximum total stem length exceeded 6m. The plant produced large (500–800 mg per fruit), but small numbers of nut-like fruit (maximum, 5 fruits per rosette). Defoliation occurred almost linearly with time at a rate of 30.6 leaves m−2 day−1; annual net leaf production was estimated to be about twice as large as the seasonal maximum leaf biomass. While the number of leaves per rosette showed moderate seasonal change, rosette density, rosette area and leaf dry weight changed considerably during the year. From the negative log-log correlation between mean total leaf dry weight per rosette and rosette density, density-dependent rosette growth was assumed. The cause of the wide spread of this species in aquatic habitats is briefly discussed in terms of its seed size and morphology.

Effects of light intensity on the growth and buoyancy of detachedElodea nuttallii (Planch.) St. John during winter

The influences of light intensity on the growth and buoyancy of detachedElodea muttallii (Planch.) St. John during winter were examined under controlled experimental light conditions. Light was controlled by mesh-screens at five levels ranging between 0.3 and 51% of the aerial full sunlight in an outdoor pond. Growth of detached segments was compared with respect to shoot and root lengths, dry weight and starch content in tissues. Buoyancy of segments at each light level was evaluated by percentage frequency of floating segments.Critical light intensity for the winter growth was estimated as ca. 4.5% of the aerial full sunlight. Most segments at light levels lower than 4.5% had been floating in water since the early period of the experiment, while all segments at light levels higher than 17% had been sinking to the bottom until water temperature became higher than 10 C. The data on segment buoyancy and tissue analysis for starch content showed an inverse relationship between percentage frequency of floating segments and starch content in tissues. These results suggest that detached segments in nature could escape from the photosynthetically unsuitable regions by reduced specific gravity caused by the consumption of starch, and establish themselves only if they could arrive at a safe-site where light conditions are sufficient to accumulate photosynthate.