Geomorphic legacy controls macrophyte distribution within and across disconnected floodplain lakes

Abstract

Summary Degradation of floodplains continues with an increasing number of floodplain lakes disconnected from the fluvial dynamics of rivers. Limited understanding is available as to how historical geomorphic formation processes (i.e. geomorphic legacy) determine contemporary ecosystem structure and function. We tested the hypothesis that geomorphic legacy mediates morphometry and results in heterogeneity of macrophyte distributions in disconnected floodplain lakes. The distribution of macrophyte cover was examined in relation to environmental factors, including water nutrient level, morphometry of lakes and patch shelter level across and among three types of lakes along the Ishikari River, Japan. Artificial lakes (isolated by channelisation), natural oxbow lakes and marsh lakes have been disconnected for more than 40 years from natural flood pulses because of dyke construction. The presence of macrophytes (in 5 × 5 m areas) was predicted well by a combination of local water depth and bed slope. Lake average depth, higher values indicating lakes that are more deeply incised with a steeper‐sloped littoral zone, had the strongest and most negative influence on total macrophyte cover across lakes. Cover was least in artificial lakes because of greater average depth. Predicted area of macrophyte cover was significantly less than occupied by actual cover in artificial lakes compared with other lake types. Macrophyte cover in artificial lakes was particularly vulnerable to external factors such as waves and wind. This study underscored the significance of geomorphic legacy in explaining a large proportion of heterogeneity of total macrophyte cover in the study lakes. Artificial lakes did not have the macrophyte habitat quality of natural lakes. When lake morphometry needs to be altered, local conditions as well as patch‐scale properties should be carefully examined in the light of the geomorphic legacy left by dynamic river–floodplain interactions.

Morphometry-Driven Divergence in Decadal Changes of Sediment Property in Floodplain Water Bodies

“…Currently, the major land use types in the Ishikari River watershed include forest, agricultural, and urban areas, the largest being forest and the smallest being urban areas [48]. Historically, the study area formed a vast wetland underlain by peat land (peat mire) [13,20]. Peatlands were largely transformed to agricultural areas between the 1950s and 1960s [20,26].…”

“…Peatlands were largely transformed to agricultural areas between the 1950s and 1960s [20,26]. Moreover, flood levees were constructed along both sides of the river together with extensive channel-straightening river works to protect farmlands from natural flooding and to consolidate fragmented farmlands [13,26]. Consequently, most of the peatlands disappeared and the most dominant land use type in the surrounding area of FWBs were paddy fields for rice and crop lands for wheat [20].…”

“…Lentic water bodies found in floodplains (floodplain water bodies; hereafter FWBs) include various types such as natural and artificial oxbow lakes, and back marsh in contemporary human-altered landscape [13]. These water bodies have continued providing important functions for living organisms, making floodplains a biodiversity hotspot [14,15].…”

“…Third, the morphometric features of FWBs such as depth and size of surface water can have decisive effects on the functionality of an entire ecosystem, which include generations of heterogeneity in sediment deposits via changes in quality and quantity of primary producers. For example, shallow water tends to provide a more suitable habitat for macrophytes and thus may promote plant succession and deposits of detritus that may differ from those originating from phytoplankton [13,29].…”

“…The Ishikari floodplain is a highly heterogeneous system which contains different types of FWBs such as marsh and natural oxbow lakes with high diversity in morphometry [13,42]. It also harbors the remnants of its historical landscape of this kind in Japan.…”