Addressing the trophic status in urban ponds: An evaluation of current trophic state indexes

Abstract

Ponds are generally understudied. Quality problems in urban freshwaters canarise from eutrophication, and trophic status classification gives information related to their ecological situation. Ponds may strongly differ from larger lakesfor which most current trophic status classification methods have been developed. This paper seeks to compare and contrast the suitability of six trophicstatus indexes (TSI) and OECD trophic classifications system currently used toevaluate the eutrophication level of urban ponds in the Subtropical region. Thetrophic status was evaluated based on total phosphorus (TP), soluble reactivephosphorus (SRP), total nitrogen (TN) and phytoplankton chlorophyll a (Chl a) in12 ponds placed in different cities from the Pampean region (Argentina), in thewarm and cold seasons. Our results demonstrate that there was no relationshipbetween pond size (0.08–2.45 ha) and trophic status. TSIs estimated with Chla showed significant differences between seasons and the range oligotrophichypertrophic of trophism was encountered. Conversely, TSIs estimated with nutrient concentrations (TP, SRP, TN) did not. The pond´s classification in trophiclevels fell in the eutrophic-hypertrophic extreme, and only considering TN/TPbroadened the trophic range. No relationship was encountered between Chl aand P; however, Chl a and TN/TP were positively correlated during the warmseason. Lower trophic levels regarding TSI (Chl a) were generally associatedwith a dense floating-macrophyte cover. We suggest that the TSI consideredshould correspond to the latitudinal region in which the ponds are located. Ourresults indicate that it appears restrictive to consider only P contents in pondsto assess eutrophication. Under the proposed framework, the key issues for thestudy of pond trophic classification will be to include not only P but N concentrations and TN/TP. Also, phytoplankton Chl a concentration jointly with floatingmacrophytes biomass/coverage should be considered as diagnosis parameters.