Hai-Jun Wang

Macrozoobenthos in Yangtze floodplain lakes: patterns of density, biomass, and production in relation to river connectivity

Abstract A systematic investigation of macrozoobenthos was conducted in Yangtze floodplain waters to reveal patterns of density, biomass, and production in relation to river connectivity. In the Yangtze-connected lakes, 78 taxa belonging to 33 families and 62 genera were identified. Macrozoobenthos density was 327 individuals/m2, biomass was 1.40 g dry mass/m2, and production was 3.23 g dry mass m−2 y−1. The assemblages were characterized by high diversity, high production, and high bivalve-filterer abundance. The key factor determining the macrozoobenthic assemblages was river connectivity. As river connectivity increased, 3 types of response patterns were observed: 1) density, biomass, and production of collector-filterers (mainly Bivalvia), shredders (e.g., Stictochironomus), and predators (e.g., Dytiscidae) showed unimodal changes, i.e., first increased and then decreased; 2) density, biomass, and production of collector-gatherers (mainly Tubificidae and Chironomidae) decreased continuously; and 3) density of scrapers (mainly Gastropoda) decreased, whereas their biomass and production changed unimodally. At an intermediate level of river connectivity, biomass and production of total macrozoobenthos reached maxima, whereas density decreased with increasing river connectivity. Previous research showed that α diversity of zoobenthos also peaks at moderate connectivity with rivers. Therefore, to maintain high productivity as well as high biodiversity in the Yangtze floodplain, protecting the remnants of river-connected lakes and linking disconnected lakes freely with the mainstream are crucial.

Macrozoobenthic assemblages in relation to environments of the Yangtze-isolated lakes

Eutrophication can shift lakes from a clear, macrophyte-dominated state to a turbid, algae-dominated state, and different habitat condition supports different fauna. Macrozoobenthos are good indicators of water environment, and studies on macrozoobenthic assemblage characteristics can help us to know which state a lake is in, thus provide the basis for its eutrophication control. In this study, a systematic investigation on macrozoobenthos was conducted in 17 Yangtze-isolated lakes to explore the macroecological laws of macrozoobenthic assemblages. Detrended correspondence analysis (DCA) revealed that variance of benthic assemblage structure occurred in two types of lakes. In macrophytic lakes, altogether 51 taxa of macrozoobenthos were identified. The average density and biomass of total macrozoobenthos were 2231 individuals·m−2 and 1.69 g dry weight·m−2, respectively. Macrozoobenthic assemblage was characterized by dominance of scrapers (i.e. gastropods). In algal lakes, altogether 20 taxa of macrozoobenthos were identified. The average density and biomass of total macrozoobenthos were 2814 individuals·m−2 and 1.38 g dry weight·m−2, respectively. Macrozoobenthic assemblage was characterized by dominance of collector-gatherers (i.e. oligochaetes). Wet biomass of submersed macrophytes (BMac) and phytoplankton chlorophyll a concentration (Chl a) were demonstrated as the key factor structuring macrozoobenthic assemblages in macrophytic and algal lakes, respectively.

Does the responses of Vallisneria natans (Lour.) Hara to high nitrogen loading differ between the summer high-growth season and the low-growth season?

Loss of submersed macrophytes is a world-wide phenomenon occurring when shallow lakes become eutrophic due to excess nutrient loading. In addition to the well-known effect of phosphorus, nitrogen as a trigger of macrophyte decline has received increasing attention. The precise impact of high nitrogen concentrations is debated, and the role of different candidate factors may well change over the season. In this study, we conducted experiments with Vallisneria natans during the growing season (June-September) in 10 ponds subjected to substantial differences in nitrogen loading (five targeted total nitrogen concentrations: control, 2, 10, 20, and 100mgL-1) and compared the results with those obtained in our earlier published study from the low-growth season (December-April). Like in the low-growth season, growth of V. natans in summer declined with increasing ammonium (NH4) concentrations and particularly with increasing phytoplankton chlorophyll a (ChlaPhyt). Accordingly, we propose that shading by phytoplankton might be of key importance for macrophyte decline, affecting also periphyton growth as periphyton chlorophyll a (ChlaPeri) decreased with increasing ChlaPhyt. Free amino acid contents (FAA) of plants tended to increase with increasing NH4 concentrations, while the relationships between FAA with growth indices were all weak, suggesting that FAA might be a useful indicator of the physiological stress of the plants but not of macrophyte growth. Taken together, the results from the two seasons indicate that although a combination of high nitrogen concentrations (ammonium) and shading by phytoplankton may cause severe stress on macrophytes, active growth in the growing season enabled them to partly overcome the stress.

Effects of high ammonia concentrations on three cyprinid fish: Acute and whole-ecosystem chronic tests

A number of studies have revealed ammonia to be toxic to aquatic organisms; however, little is known about its effects under natural conditions. To elucidate the role of ammonia, we conducted 96-h acute toxicity tests as well as a whole-ecosystem chronic toxicity test for one year in ten 600-m2 ponds. Three common cyprinids, silver carp Hypophthalmichthys molitrix Val. (H.m.), bighead carp Aristichthys nobilis Richardson (A.n.), and gibel carp Carassius auratus gibelio Bloch (C.g.), were used as test organisms. The 96-h LC50 values of un-ionized ammonia (NH3) for H.m., A.n., and C.g. were 0.35, 0.33, and 0.73mgL-1, respectively. In the ponds, annual mean NH3 ranged between 0.01 and 0.54mgL-1, with 4 ponds having a NH3 higher than the LC50 of A.n. (lowest LC50 in this study). No fish were found dead in the high-nitrogen ponds, but marked histological changes were found in livers and gills. Despite these changes, the specific growth rate of H.m. and A.n. increased significantly with NH3. Our pond results suggest that fish might be more tolerant to high ammonia concentrations in natural aquatic ecosystems than under laboratory conditions. Our finding from field experiments thus suggests that the existing regulatory limits for reactive nitrogen (NH3) established from lab toxicity tests might be somewhat too high at the ecosystem conditions. Field-scale chronic toxicity tests covering full life histories of fish and other aquatic organisms are therefore encouraged in order to optimize determination of the effects of ammonia in natural environments.

Macroinvertebrate responses to regime shifts caused by eutrophication in subtropical shallow lakes

The ecological status of subtropical floodplain lakes is threatened by eutrophication, which can shift lakes from a clear, macrophyte-dominated equilibrium state to a turbid, phytoplankton-dominated state. Such a shift is reflected in their macroinvertebrate assemblages, which may serve as good indicators of long-term changes in such lake ecosystems. We conducted a survey of benthic macroinvertebrates in 20 lakes in the Yangtze floodplain that were disconnected from the river channel to identify invertebrate response patterns along a gradient of eutrophication. Macroinvertebrate assemblages changed in parallel to the observed regime shift of a subgroup of those lakes. The most abundant groups were epiphytic invertebrates (e.g., Bithyniidae) in the macrophyte-dominated lakes and pollution-tolerant invertebrates (e.g., Tubificidae, Tanypus) in the phytoplankton-dominated lakes. Planktonic chlorophyll a concentration and wet biomass of submersed macrophytes were the key factors structuring macroinvertebrate assemblages. Macroinvertebrate taxon richness decreased along the eutrophication gradient, but density reached a minimum at a moderate eutrophication level and then increased. Scraper density (e.g., Bithyniidae) dropped abruptly at the moderate eutrophication level and did not increase again, whereas collector–gatherers (mainly Tubificidae and Chironomidae) and predators (e.g., Tanypus) increased continuously along the eutrophication gradient. We conclude that the abrupt disappearance of macrophytes during the regime shift means the loss of key habitat for biota associated with aquatic plants. Therefore, ecological restoration projects should be designed to reduce the nutrient concentration of floodplain lakes enough to allow re-establishment of a clear-water lake equilibrium dominated by aquatic macrophytes.

Can short-term and small-scale experiments reflect nutrient limitation on phytoplankton in natural lakes?

Whether it is necessary to reduce nitrogen (N) and/or phosphorus (P) input to mitigate lake eutrophication is controversial. The controversy stems mainly from differences in time and space in previous studies that support the contrasting ideas. To test the response of phytoplankton to various combinations of nutrient control strategies in mesocosms and the possibility of reflecting the conditions in natural ecosystems with short-term experiments, a 9-month experiment was carried out in eight 800-L tanks with four nutrient level combinations (+N+P, −N+P, +N−P, and −N−P), with an 18-month whole-ecosystem experiment in eight ~800-m 2 ponds as the reference. Phytoplankton abundance was determined by P not N, regardless of the initial TN/TP level, which was in contrast to the nutrient limitation predicted by the N/P theory. Net natural N inputs were calculated to be 4.9, 6.8, 1.5, and 3.0 g in treatments +N+P, −N+P, +N−P, and −N−P, respectively, suggesting that N deficiency and P addition may promote natural N inputs to support phytoplankton development. However, the compensation process was slow, as suggested by an observed increase in TN after 3 weeks in −N+P and 2 months in −N−P in the tank experiment, and after 3 months in −N +P and ~3 months in −N−P in our pond experiment. Obviously, such a slow process cannot be simulated in short-term experiments. The natural N inputs cannot be explained by planktonic N-fixation because N-fixing cyanobacteria were scarce, which was probably because there was a limited pool of species in the tanks. Therefore, based on our results we argue that extrapolating short-term, small-scale experiments to large natural ecosystems does not give reliable, accurate results.

Linking greenhouse gas emissions to urban landscape structure: the relevance of spatial and thematic resolutions of land use/cover data

ContextEmissions of greenhouse gases in urban areas play an important role in climate change. Increasing attention has been given to urban landscape structure–emission relationships (SERs). However, it remains unknown if and to what extent SERs are dependent on observational scale.ObjectiveTo assess how changing observational scales (in terms of spatial and thematic resolutions) of urban landscape structure affect SERs.MethodsWe examined correlations between 16 landscape metrics and greenhouse gas emissions across 52 European cities, through (1) systematic manipulation of spatial and thematic resolutions of the urban land use/cover (ULUC) dataset, and (2) comparison between available standard ULUC datasets with different spatial resolutions.ResultsOur analyses showed that the observed SERs significantly depend on both thematic and spatial resolutions of the ULUC data. For the 16 landscape metrics, we found diverse spatial/thematic scaling relations exhibiting monotonic, hump-shaped or scale-invariant trends. For different landscape metrics, the SERs were strongest at different spatial scales, suggesting that there is no consistent scaling relation over those observational scales.ConclusionsSERs are highly sensitive to spatial and thematic resolutions of landscape data. To avoid the problem of ‘ecological fallacy,’ important caveats should be taken for interpretations based on single landscape metrics. Particular consideration should be paid to metrics that are easily interpretable, predictable in scaling behaviors, and important for shaping SERs, such as PLAND, ED, and LPI. Systematic investigations on scaling behaviors of SERs over well-defined scale domains are encouraged in future studies linking greenhouse gas emissions and urban landscape structure.

Sustainable fisheries in shallow lakes: an independent empirical test of the Chinese mitten crab yield model

Next to excessive nutrient loading, intensive aquaculture is one of the major anthropogenic impacts threatening lake ecosystems. In China, particularly in the shallow lakes of mid-lower Changjiang (Yangtze) River, continuous overstocking of the Chinese mitten crab (Eriocheir sinensis) could deteriorate water quality and exhaust natural resources. A series of crab yield models and a general optimum-stocking rate model have been established, which seek to benefit both crab culture and the environment. In this research, independent investigations were carried out to evaluate the crab yield models and modify the optimum-stocking model. Low percentage errors (average 47%, median 36%) between observed and calculated crab yields were obtained. Specific values were defined for adult crab body mass (135 g/ind.) and recapture rate (18% and 30% in lakes with submerged macrophyte biomass above and below 1 000 g/m2) to modify the optimum-stocking model. Analysis based on the modified optimum-stocking model indicated that the actual stocking rates in most lakes were much higher than the calculated optimum-stocking rates. This implies that, for most lakes, the current stocking rates should be greatly reduced to maintain healthy lake ecosystems.