Modifications of ecological trophic structures on chemical gradients in lotic ecosystems and their relations to stream ecosystem health

Abstract

The objectives of this study were to evaluate how chemical gradients and physical habitats influence trophic dynamics and chemical tolerance in relation to ecological health, based on biological integrity model in the absence of cascading pressures (cascading theory) as a control mechanism of aquatic ecosystems. We conducted physical, chemical, and biological surveys from 76 national streams and rivers of four major watersheds during 2004Á2005 along with surveys of 80 reference streams. Maximum species richness lines (MSRLs) in reference sites vs. regular sampling sites indicated that the third-to fifth-order streams were evidently impaired in the metrics of trophic and tolerant components. These trophic modifications were linked with land-use patterns, which resulted in differences of nutrients (N, P), organic matter contents and the N:P ratios on trophic structures. Overall, these trophic factors influenced ecological health, based on multimetric index of biological integrity model in these systems, so that trophic compositions and tolerance were regulated by bottom-up hypothesis.

Roles of N:P Ratios on Trophic Structures and Ecological Stream Health in Lotic Ecosystems

“…In other words, the N:P ratios determined the sestonic CHL, which was associated with the food chain at higher trophic levels. Fish trophic compositions and tolerance guilds (14 [8]) were closely associated with the food chain and were directly affected by N:P ratios in ambient water, resulting in a modification of stream ecosystem health (based on the IBI multi-metric model, 14 [8]). Overall, the N:P ratio may be a good surrogate variable of ambient concentrations of N or P in assessing trophic linkage and diagnosing the ecological stream health in aquatic ecosystem.…”

“…Strong negative regression coefficients (R 2 = 0.64, p < 0.001) for the N:P ratios on TP were found in numerous lentic ecosystems [22,57]. These studies indicated that the N:P ratio is a key regulator of the nutrient regime, and of primary production, in ambient water [11,12,58], and is also a major indicator of ecological river health in multi-metric fish models [8]. Our results indicate that the N:P ratios in forest region (high N:P) were clearly segregated from the WTPs region (low N:P), but partial overlaps in the N:P ratios were also shown in the cropland and urban regions.…”

“…The N:P ratios were directly or indirectly associated with the trophic level of phytoplankton production (sestonic CHL; 5 , 6 [32,65]) and were determined by P rather than N, suggesting that differences in TP and N:P ratios were related to land use patterns and the location of WTPs. Furthermore, fish trophic compositions ( 14 [8]), tolerance guilds ( 14 [8]), and fish community (15 [9]) were determined by the availability of food resources (13 [66]), which are directly influenced by N:P ratios or nutrient regimes (N, P). In other words, the N:P ratios determined the sestonic CHL, which was associated with the food chain at higher trophic levels.…”

“…Furthermore, the importance of 16N:1P molar ratios as well as ambient nutrient concentrations in aquatic ecosystems [4], and suggested their key roles in primary production, nutrient cycling, resource competition, and animal growth in the systems, in spite of partial limitation on biological unavailability of some forms of nutrients. The various roles of N:P ratios, therefore, have been frequently tested in various trophic linkages as criteria of nutrient limitation on phytoplankton growth (i.e., bluegreens; Smith [5]), invertebrate compositions [6], consumer-resource [7], and fish trophic guilds [8,9], and the ecological stream health [8]. In other words, the N:P ratios determined the production of primary producers [10], and this influenced the compositions and guilds in the higher trophic consumers.…”

“…Therefore, these ratios have been used as a stoichiometric determinant in ambient water for trophic allocation of low-level organisms [5,12] through to higher trophic level organisms [6,7,9]. Previous omnivorous fish due to changes in their feeding resources (in accordance with water chemistry [8,40]). These findings suggest that nutrients and stoichiometric N:P ratios may alter species compositions, tolerance level, and trophic compositions in aquatic ecosystems.…”