MAPPING SPATIOTEMPORAL TRENDS IN THE ABUNDANCE AND DISTRIBUTION OF MACROPHYTES IN HONGZE LAKE

GUO Chuan-Bo， LI Wei， ZHANG Ying-Xue， ， XIA Wen-Tong， ， XIN Wei， CHEN Yu-Shun， and LI Zhong-Jie，

(1. State Key Laboratory of Freshwater Ecology and Biotechnology， Institute of Hydrobiology， Chinese Academy of Sciences，Wuhan 430072， China; 2. University of Chinese Academy of Sciences， Beijing 100049， China)

Abstract: A comprehensive investigation on macrophyte community in Hongze Lake was conducted seasonally from May 2010 to February 2011. Overall， twelve species representing eight families of macrophytes were identified in Hongze Lake， including nine species of submerged plants， two species of floating-leaved plants， and one species of emerging plant. In general， Potamogeton malaianus， P. maackianu， P. pectinatus and P. crispus were the four dominant species throughout the whole year， the highest biomass of macrophytes was presented in autumn， followed by summer and winter， while spring had the lowest biomass of macrophytes. Based on field data， we used kriging interpolation in ArcGis to map the spatiotemporal distribution of the entire macrophyte community as well as each of the four dominant species. From the GIS maps we observed that the northern area of the lake， namely the Chengzihu region， had the highest biomass of macrophytes potentially as a result of better water quality and greater transparency. Potential factors that affected the community structure， biomass， and distribution patterns of macrophytes considerably were then discussed.The results of this study illuminate the need for more information on the role and importance of aquatic macrophytes in shallow lake ecosystems. Conservation of macrophytes should be taken to maintain the lake ecosystem health.

Key words: Macrophyte community; GIS and GPS; Hongze Lake; Spatiotemporal patterns; Shallow lakes

Macrophytes play an important role in the structure and functioning of shallow freshwater ecosystems[1—3]. They serve as a base of most aquatic foodwebs， and also produce food for biota， provide refugia for other organisms[4]， anchor soft bottom sediments， remove suspended particles and nutrients[4]，and contribute to the promotion and maintenance of aquatic foodwebs and ecosystem services[5，6]. Macrophytes also contribute to the general fitness and diversity of aquatic ecosystems by serving as indicators for water quality and aiding in nutrient cycling[7，8]. As a result of their significance， macrophytes are commonly viewed as one of the most important foci in shallow lake ecosystems. Generally， the function of macrophytes in shallow lake ecosystems is related to their structural attributes such as species composition，distribution， abundance， and diversity. These attributes which in turn rely on various environmental factors including water level， water temperature， substrate composition， disturbance， competitive interactions， herbivory， epiphyte loading， water quality and sediment nutrients[3，9—11]. Therefore understanding and quantifying the community structure and spatiotemporal patterns of macrophytes is indispensable for integrated management practices and aquatic conservation in shallow lake ecosystems. An example of a macrophyte-driven shallow water ecosystem is that of Hongze Lake.

Hongze Lake (33°06′—33°40′N， 118°10′— 118°52′E)， which lies in the middle reach of the Huaihe River， is the fourth largest freshwater lake in China with a surface area of 1597 km2， a mean water depth of 1.7 m[12]. Hongze Lake is also the largest storing reservoir and water channel on the east route of the China’s South-North Water Division Project (SNWDP)[13]. Hongze Lake ecosystems provide multiple benefits that are fundamental to human wellbeing， including commercial navigation， flood control， commercial fishing， aquaculture， biodiversity conservation， and tourism in eastern China[12，13]. However，Hongze Lake has also experienced numerous natural and artificial disturbances in recent years， such as regional climate change， high nutrients inputs， water conservancy construction， recreational and commercial fishing， and aquaculture. As a result of these disturbances， aquatic macrophytes have seen great declines， specifically the biomass and species diversity has decreased whilst distributional areas shrunk[13，14].These alterations have the potential to affect the distribution patterns and community structure of other organisms which can ultimately， affect the overall health of the lake ecosystem[15].

However， very little knowledge about the spatial and temporal dynamics of macrophytes in Hongze Lake exists， mainly due to the fact that large-scale and long-term field monitoring efforts for macrophytes in such a large lake presents numerous challenges. Previous studies of macrophytes in Hongze Lake have only been conducted with very limited datasets either on a small spatial scale or during a single season[14，16].The current study was conducted by integrating GPS and GIS technologies with field monitoring data to simulate the community structure and spatiotemporal patterns and dynamics of macrophytes in the whole Hongze Lake. The main objective of our study was to map the distribution patterns and seasonal dynamics of macrophytes community structure in Hongze Lake across broad spatial and temporal scales. The results of this study will benefit the conservation of macrophytes resources in not only Hongze but other shallow lakes as well.

1 Materials and Methods

1.1 Study area

Hongze Lake is the fourth largest freshwater lake in China with a surface area of 1597 km2， and a mean water depth of 1.7 m[12]. The rivers draining into Hongze Lake are located primarily along the western bank of the lake. Among them， the Huaihe River is the largest， and contributes 87% of the total inflow to the lake[12]. Hongze Lake is a transitional lake， meaning the water levels of the lake often undergo large annual and seasonal changes[17]. The Hongze Lake ecosystem provides multiple benefits to various stakeholders. Hongze Lake acts as a navigation junction，aids flood-and-drought resistance， provides fishing and aquaculture opportunities， as well as supports biodiversity conservation and tourism in eastern China[12，13].

1.2 Sampling procedure

Four field surveys were conducted in Hongze Lake during 2010－2011 in spring (May)， summer(August)， autumn (November) and winter (February)respectively. Finally， a total of 631 sites， each consisting of a 0.125 m2quadrat， were randomly selected in the whole lake according to the lake morphology and habitat heterogeneity. Seasonaly， we sampled 173，173， 130 and 155 sites during spring， summer， autumn and winter respectively. Throughout sampling，we avoided selection of area near intensive block nets in the lake (Fig. 1). Within each 0.125 m2sampling quadra in each of the sampling site， all macrophytes were uprooted with a bottom sampler. All macrophytes were then cleaned， identified into species following Cook[18]and then weighed to the nearest 0.1 g wet weight in the field immediately. The geographic coordinates of each sampling site were recorded using a handling GPS unit (Garmin eTrex 301).

1.3 Data processing and GIS mapping

The relative biomass (Bri)， frequency of occurrence (Fi) and degree of dominance (Di) were calculated according to the following equations[19]:

WhereBriis the relative biomass of the species i，Biis the biomass of the species i，Btis the total biomass of all the species;Fiis the frequency of occurrence of species i，NiandNtis the occurrence number of species i and the total sampling sites， respectively;Diis the degree of dominance.

The spatiotemporal patterns of the total macrophytes biomass and four dominant macrophyte were then mapped using Arcgis 10.1 (ESRI) using the Kriging interpolation utility on a map of Hongze Lake that was modified from Google Earth (Google Earthtm).

2 Results

2.1 Community structure of macrophytes

A total of twelve species belong to eight families of macrophytes were identified from the whole Hongze Lake. Collections included nine species of submerged plants (Potamogeton malaianus，Myriophyllum spicatum，P. maackianus，P. pectinatus，Hydrilla verticillata，Ceratophyllum demersum，Elodeanuttallii，P. crispus，Vallisneria natans)， two species of floating-leaved plants (Eichhornia crassipes，Trapa bispinosa) and one species of emerging plant(Miscanthus sacchariflorus). The greatest biomass of macrophytes were observed during autumn， followed by summer and winter， while spring contained the lowest macrophyte biomass (Fig. 2). In particular，high biomasses ofP. crispusobserved mostly during spring and winter had disappeared by summer， while biomasses ofE. nuttalliiandM. sacchariflorusincreased only during spring (Tab. 1; Fig. 2).

Fig. 1 Location and distribution of sampling sites in four seasons in Hongze Lake

Fig. 2 Seasonal variations of species composition of macrophytes community in Hongze Lake

Macrophyte communities in Hongze Lake were dominated byP. malaianus，P. maackianu，P. pectinatusandP. crispusthroughout the year (Fig. 2).The dominant species showed significant difference among the four seasons:P. maackianusandP. crispusdominated the macrophyte community during Spring withDivalues of 32.1% and 20.1%， respectively. During Summer， communities shifted more towardsP. malaianus(21.3%)，C. demersum(15.1%)，P. pectinatus(9.3%) andT. bispinosa(8.9%) which dominated the community. By autumn， communities had transitioned over toP. malaianus(28. 5%) andP.maackianus(23.7%). AndP. maackianus(18.7%)，P.malaianus(18.3%) andP. pectinatus(13.7%) dominating the macrophyte communities in winter (Tab. 1).

2.2 Spatiotemporal distribution patterns of macrophytes

Using the GIS simulation， the spatiotemporal distributions of macrophyte biomass was mapped in a set of quadruple lattices. The greatest biomass of macrophytes was distributed in the northern part of the Hongze Lake， where large areas of macrophytes existed during summer， and smaller patch of high biomass was found in autumn (Fig. 3).

Distribution patterns ofP. malaianusvaried significantly across seasons (Fig. 4).P. malaianushad lowest biomass in spring and highest in autumn (Fig. 4).Although the western， northern， and northeastern parts of Hongze Lake were occupied byP. malaianusin summer and autumn， distributions were limited to the central and northeastern parts of the lake by winter (Fig. 4). High-biomass patches ofP. pectinatuswere found in the northern and northeastern part of Hongze Lake during summer， but were limited to only the northern part of the lake in winter (Fig. 5).P. pectinatusbiomasses also were high in the western and northeastern part of the lake in autumn， though biomasses were never as high as those observed during summer or winter (Fig. 5).P. maackianusbiomass was elevated in the northern part of the lake during spring and autumn， but was limited to the western and central parts of the lake during winter (Fig. 6).Generally， significant high biomass patch can be also seen in the central and west portion of the lake in spring and winter. During summer，P. crispuscould not be detected in the lake (Fig. 7).

3 Discussions

3.1 Community structure of macrophytes

Tab. 1 Seasonal variation of macrophyte quantitative characteristics in Hongze Lake

The biomass of macrophytes was historically high in Hongze Lake prior to the construction of a dam between Hongze Lake and Huaihe River[16]. Significant degradation of macrophytes communities was observed following dam construction， such that both biomass and diversity were decreased. In 1960， there were 28 species of macrophytes were recorded， compared to 29 species in 1981[20]. However， Liu，et al.[16]reported 25 species of macrophyte species in the early 2000s. All of these figures sharply contrast with the current study， where only 12 species were recorded. Although these suggest that macrophyte diversity in Hongze Lake has declined sharply， results also partly variations in species collection area and sampling methods used across studies. In the current study， we focused on macrophytes in the open-water areas that excluded littoral zones and border wetlands that would likely support more species. In addition，there was a large area in the lake being used for enclosure aquaculture， which was not accessible to our sampling. Thus， our current estimate of 12 macrophyte species is likely an underestimate. Liu，et al.[16]revealed thatP. malaianuswas the dominant species in Hongze Lake during their sampling in the early 2000s， which was consistent with our current findings. However， we also argue that the dominant species exhibited significant seasonal variation. For example，P. maackianusandP. crispusdominated the macrophyte community in spring，P. malaianus，C. demersum，P. pectinatusandT. bispinosadominated the summer community，P. malaianusandP. maackianusdominated the autumn community，P. maackianus，P. malaianusandP. Pectinatusdominated the winter community. Overall，P. malaianus，P. maackianu，P. pectinatusandP. crispuswere the four most dominant species in Hongze Lake. Some of the seasonal variation was due to basic life histories. For instance，P. crispuswas only observed in spring and winter because this species dies off gradually during spring and summer， but will sprout and grow during winter[21].

Fig. 3 Distribution of total biomass (g/m2) of macrophytes in Hongze Lake by GIS simulation using kriging interpolation utility

Fig. 4 Distribution of the biomass (g/m2) of P. malaianus in Hongze Lake by GIS simulation using kriging interpolation utility

Fig. 5 Distribution of the biomass (g/m) of P. pectinatus in Hongze Lake by GIS simulation using kriging interpolation utility

Fig. 6 Distribution of the biomass (g/m2) of P. maackianus in Hongze Lake by GIS simulation using kriging interpolation utility

Fig. 7 Distribution of the biomass (g/m2) of P. crispus in Hongze Lake by GIS simulation using kriging interpolation utility

3.2 Possible factors affecting distribution patterns

There is ample evidence in the literature that many factors have affected the distribution， and species composition of macrophyte communities. These factors include light， water temperature， water quality changes and nutrient enrichment， sediment composition， and water level fluctuations[22—24]. Light and temperature within the context of water depth， season，and latitude are of paramount importance in dictating macrophyte distributions， thereby indirectly influencing lake productivity and macrophyte species composition[25]. The distribution patterns of macrophytes are largely driven by water clarity， which affects the amount of light reaching the lake bottom where plants root[26—28]. Water quality changes and associated nutrient enrichment often cause considerable variation in the species richness， composition， and density for a variety of aquatic vegetation species[3，29，30]. In the current study， the northern and northeastern parts of Hongze Lake functioned like a “closed” system(Chengzihu area) in that water quality was in generally good condition with high transparency[31]. Not surprisingly， more macrophyte species that exhibited high biomasses throughout the whole year were distributed in this area of the lake. However， in the central and southern parts of Hongze Lake， high nutrient loading related to enclosure aquaculture and urbanization adjacent to the lake greatly affected water qua lity. Thus， few macrophyte species with generally low biomasses were collected in those areas[31].

Hydrological variations as related to fluctuating water levels also affect the distribution of macrophytes in lakes[32]. In particular， water level fluctuations directly and indirectly affect the loading of nutrients and sediments into the receiving lake[33]. It is well documented that the water-level variations affect lake limnology， which in turn， affects species zonation， distribution， biomasses， and richness of macrophytes on decadal time scales[34]. Hongze Lake formed as a reservoir in the 1950s when Sanhe Dam was constructed between the lake and the Huaihe River. As a result， base water levels have increased from 10.6 to 12.4 m during the time[16]; by the 1990s，water levels were further increased to 13.0 m[35]. The high frequency of historical water level changes has likely greatly affected macrophytes in Hongze Lake[34].

Another possible effect has been the construction of the SNWDP water diversion project， which is suspected to have significantly affected the composition and distribution of macrophytes in Hongze Lake.This water-diversion project has altered water levels in and water flows to Hongze Lake， thereby increasing nutrient loads， and reducing water transparency.Finally， overfishing and enclosure aquaculture along the shorelines of Hongze Lake have likely affected macrophytes directly by reducing herbivore grazing，and indirectly from external nutrient inputs that have increased water turbidity and nutrient loading[13，16].

Overall， more attention is needed towards the macrophyte communities in Hongze Lake due to many factors that could have profound impacts on the distribution and composition. Conservation and restoration strategies also should be considered given the significance of macrophyte communities in shallow lake ecosystems.