Resposta do perifíton aos pulsos de enriquecimento em níveis crescentes de fósforo e nitrogênio em represa tropical mesotrófica (Lago das Ninféias, São Paulo)

Abstract

O estudo visou a (a) caracterizar as principais alteracoes estruturais da comunidade de algas perifiticas em condicoes de enriquecimento por pulsos de nitrogenio e fosforo (b) identificar a formacao de guildas de especies indicadoras; (c) avaliar a contribuicao do perifiton em relacao a outros compartimentos biologicos na retencao do fosforo. O experimento foi realizado em represa rasa tropical a partir de mesocosmos com controle de fluxo de agua, onde foram colocadas laminas de vidro para o perifiton, 78 L de agua da represa e 100 individuos de Ricciocarpus natans. O delineamento constou de controle e tres tratamentos (n = 3), com niveis crescentes de nutrientes (RN/P molar = 44): T1 - 100 ÊgP L-1 e 2000 ÊgN L- 1; T2 - 200 ÊgP L-1 e 4000 ÊgN L-1; T3 - 400 ÊgP L-1 e 8000 ÊgN L-1. Dezessete amostragens semanais foram realizadas durante 64 dias, sendo uma antes da adicao dos sais (preenriquecimento) e outra 24h apos adicao (pos-enriquecimento). A ACP separou as observacoes do controle e do pre-enriquecimento daquelas do pos-enriquecimento. Pela OECD, os tratamentos foram classificados em mesotrofico (C), eutrofico (T1, T2) e hipereutrofico (T3). O incremento de biomassa perifitica foi primordialmente direcionado pelas cargas crescentes de nutrientes e menos pelos pulsos de enriquecimento. O efeito cumulativo dos pulsos sobre a biomassa fitoplanctonica foi bem menor, sendo apenas evidente no T3 para as macrofitas. A composicao quimica da biota nao respondeu ao aumento das cargas de nutrientes e as comunidades autotroficas permaneceram P-limitadas. Em relacao ao balanco de massa, o seston foi o principal sequestrador de P, seguido pelo complexo macrofitas-perifiton e pelo perifiton. Entretanto, parte substancial de P ficou retida, muito provavelmente, no sedimento e sua microflora associada. Foram caracterizadas tres guildas indicadoras das condicoes ambientais.

The study aimed at (a) characterizing the main periphytic algae community structural modifications under nitrogen and phosphorus pulses enrichment conditions; (b) identifying the formation of indicating species guilds; and (c) evaluating the periphyton contribution towards phosphorus retention facing other biological compartments in the system. Experiment was carried out in a tropical shallow reservoir using water flux controled mesocosms where microscope glass slides were placed for periphyton attachment, 78 L reservoir water and 100 Ricciocarpus natans individuals. Experiment design included one control and three treatments (n = 3) with different nutrient concentrations (molar N/P ratio = 44): T1 - 100 ìgP L-1 and 2,000 ìgN L-1; T2 - 200 ìgP L-1 and 4,000 ìgN L-1; T3 - 400 ìgP L-1 and 8,000 ìgN L-1. Seventeen weekly samplings were performed during 64 consecutive days, one of them carried out before the enrichment (pre-enrichment) and the other one 24 hr after P addition (postenrichment). Principal Component Analysis separated observations of the control and preenrichment from those of the post-enrichment. According to OECD, treatments were classified as mesotrophic (C), eutrophic (T1, T2) and hypereutrophic (T3). Periphytic biomass increase was mostly directed by the increasing nutrient loads, and less by the enrichment pulses. Cumulative effect of pulses on the phytoplankton biomass was much lesser and evident only at T3 treatment for the macrophytes. Chemical decomposition of the biota did not respond to the increase in nutrient loads, and the autotrophic communities remained P-limited. Regarding to the mass balance, seston was the main P sequestrator, followed by the complex macrophytes-periphyton and the periphyton. Substantial part of P remained, however, trapped much probably in the sediments and its associated microflora. Three guilds indicative of environmental conditions were identified.