Abstract

SHORT COMMUNICATION

Temporal variations in the primary productivity of Eleocharis acutangula (Cyperaceae) in a tropical wetland environment

Gustavo Gomes ChagasI;^* * Corresponding author: chagas.gg@gmail.com ; Giselli Martins de Almeida Freesz^I; Marina Satika Suzuki^I

^IUniversidade Estadual do Norte Fluminense Darcy Ribeiro, Centro de Biociências e Biotecnologia, Laboratório de Ciências Ambientais, Av. Alberto Lamego, 2000, Pq. Califórnia, 28013-600 Campos dos Goytacazes, RJ, Brazil.

ABSTRACT

Wetland vegetation typically includes aquatic macrophytes with high primary production capacities. The present study investigated how hydrological variations affect biomass allocation and primary productivity in the emergent macrophyte Eleocharis acutangula (Roxb.) Schult. Eleocharis acutangula ramets were collected from the Campelo Lagoon flood plain (21°39'S, 41°12'W and 21°37S, 41°11'W) between March/2005 and February/2006. This region experienced an unusually short rainy period between November/2005 and February/2006 that generated atypically high primary production levels (128gDWm-2month-1) and total biomass gains (447gDWm-2) in May and June/2005 respectively. Our data indicated that primary production and biomass allocation were strongly influenced by variations in wetland water levels and that macrophytes quickly invested in biomass accumulation when surface water levels rised.

Keywords: aquatic macrophyte, biomass allocation, primary production, wetland

INTRODUCTION

Wetlands are transition regions between terrestrial and aquatic environments where the water table is very close to the surface or the soil is actually covered by standing water, and they are considered the most productive ecosystems on the planet (Mitsch & Grosselink 2007). Wetland vegetation typically consists of aquatic macrophytes with high primary productivity capacities (Esteves 1998).

Hydrological conditions are of upmost importance in maintaining the structure and functioning of wetlands, affecting nutrient availability, soil anaerobiosis and salinity, and determining species composition and richness (Amezaga et al. 2002). When hydrological patterns remain regular, the ecological integrity of these ecosystems can persist unchanged for many years (Mitsch & Gosselink 2007).

Seasonal variations in primary productivity and in the organic and inorganic compositions of aquatic macrophytes are very apparent in temperate regions (Fernández-Alaez et al. 2002, Fox et al. 2008). These variations are not always as evident in tropical regions (Esteves et al. 2005, Freesz 2007), although it is possible to detect seasonal changes in the chemical compositions and biomasses of aquatic macrophytes in environments that experience marked variations in their water levels (Da Silva & Esteves 1993, Santos & Esteves 2002). The primary production rates of aquatic macrophytes in tropical environments are known to be sensitive to variations in temperature and light levels, as well as to water and nutrient availability (Neue et al. 1997).

The present study evaluated primary productivity and biomass allocation in the emergent macrophyte Eleocharis acutangula (Roxb.) Schult. in northern Rio de Janeiro State, a region with numerous lentic environment that experience both periodic flooding and drought conditions.

MATERIAL AND METHODS

This study was carried out in a wetland ecosystem in the municipality of Campos dos Goytacazes in northern Rio de Janeiro State, Brazil. This wetland site is part of the seasonally flooded area of the Campelo Lagoon (21°39'S, 41°12'W and 21°37S, 41°11'W). The site is narrow and elongated, and parallel to a lagoon that overflows and floods it during the rainy season.

This site may accumulate up to 80cm of water during the rainy period (October-March), although it will fully drain during the dry period (June-September), leaving only boggy soils. The area is colonized by permanent stands of Typha domingensis Pers. and seasonal stands of Nymphaea sp., Eleocharis acutangula, and Chara sp. The region has a tropical dry to sub-humid climate (Fiderj 1978) with a total annual rainfall of approximately 1000mm that is fairly well-distributed heat throughout the year, but with higher precipitation rates during the austral summer and dry winters. Average temperatures remain between 20 and 30°C.

Eleocharis acutangula was sampled on a monthly basis from March/2005 to February/2006. Ten 0.25m2 plots were established along a 50 meter transect at intervals of 5 meters, and all of the macrophytes present inside the plots were cut at sediment level and placed in plastic bags. The water levels in the wetland areas were measured when the macrophytes were harvested. The samples were washed in running water in the laboratory and separated into chlorophyllous portions (ramets with more than 50% of their length having chlorophyllous tissue) and debris (less than 50% of the ramet length containing chlorophyllous tissue). The samples were dried at 70°C to a constant weight, and the biomass expressed as average dry weight per square meter (gDWm-2).

The primary aerial production (PAP) was determined according to Smalley (1959). Positive changes in living biomass between two consecutive samplings periods were noted and biomass losses during the time intervals were considered to consist of debris biomass.

Spearman's coefficient (P = 0.05) was used to evaluate the degrees of correlation between the variables.

RESULTS

Reductions in the total aerial biomass of the macrophyte were observed to follow reductions in water levels in the wetland site. The highest biomass values were observed between April and July, as well as in September, when they exceeded 400gDWm-2 (figure 1A). A small increase in total biomass was observed between April and September/2005, with lower water depths, despite reductions in chlorophyllous biomasses and increases in debris biomass. The correlation between chlorophyllous biomass and water level was highly significant (Spearman P>0.0001).

The total aboveground biomass of E. acutangula varied from 21gDWm-2 (February 2006) to 447gDWm‑2 (July 2005). The highest chlorophyllous biomass value (which varied between 13.6 and 305gDWm-2) was observed in April 2005 (figure 1B) at the end of the rainy period. The maximum senescent biomass was observed in September 2005 (291gDWm-2).

Primary aerial production increased only in April, May and July/2005, when the wetland water levels were above 50cm (figure 1C). The highest PAP value was observed in May/2005 (128gDWm-2month-1) and the lowest positive value was observed in April/2005 (45gDWm-2month-1). Primary production values were zero or negative between August/2005 and February/2006, although the method does not account for these values. The annual primary production was estimated at 282gDWm-2 year-1.

DISCUSSION

The hydrochemistry and dynamics of wetland environments are strongly affected by annual variations in rainfall (Mitsch & Gosselink 2007). The leaves of aquatic macrophytes sprout throughout the year in tropical environments, although there is evidence that water-level fluctuations induce changes in aquatic macrophyte productivity and biometry (Froend & McComb 1994, Penha et al. 1999, Santos & Esteves 2004). The results of the present work corroborate those of previous studies indicating that variations in Eleocharis acutangula aerial biomass are strongly associated with variations in wetland water levels (figure 1A).

The variation seen in chlorophyllous biomass follow the developmental dynamics of E. acutangula ramets in these wetlands (figure 1B). Despite the fact that primary aerial production values decreased with receding water levels in March, they were higher in April and May/2005. Increases in the proportion of chlorophyllous biomass to total biomass were observed in December/ 2005 following a slight increase in wetland water levels (figure 1A), clearly indicating that these plants invest in growth, biomass allocation, and ramet development in response to variations in wetland water levels, with increasing water levels resulting in increases in the total aerial biomass and chlorophyllous biomass.

The populations of E. acutangula demonstrated increasing senescence as water levels decreased, and these plants will even disappear when the sediments become fully exposed. Variations in the wetland water levels cause differential disturbances in the different macrophyte populations in the ecosystem, but even the complete exposition of the sediment surface does not result in the total elimination of the aerial biomass of Typha domingensis, for example. Previous studies of T. domingensis showed that this macrophyte can grow in saturated soils without any standing water (Esteves & Suzuki 2008).

The present study determined that the period of aquatic macrophyte growth in tropical wetlands is related to increases in standing water levels, and not to seasonal temperature variations, as has been reported by other authors (Thomaz & Bini 1998, Pezzato & Camargo 2004).

The biomass data obtained in the present study was similar to that reported in other studies: Santos & Esteves (2004) noted that the total biomass values of Eleocharis interstincta (Vahl) Roemer varied from 20gDWm-2 to 339gDWm-2 in the Jurubatiba coastal lagoon; Enrich‑Prast et al. (2002) reported that the total biomass values of Oryza glumaepatula Steud. varied from 7.1gDWm-2 to 467gDWm-2 in a non-impacted area of Lake Batata in the Amazon Region. Interestingly, the highest biomass values in the present study were observed during an atypical dry period, while these other studies reported maximum biomass values during the rainy period when water levels increase.

The annual PAP value observed for E. acutangula in the present study (282gDWm-2year-1) was lower than that observed for Eleocharis interstincta by Santos & Esteves (2002) in the Jurubatiba coastal lagoon (PAP = 1012gDWm-2year-1), which may be related to low productivity during the 2005/2006 rainy period and clearly indicates that hydrological variations are the most important factors influencing the dynamics of Eleocharis acutangula productivity.

In conclusion, reductions in wetland water levels produced extensive mortality of Eleocharis acutangula populations, and our data indicates that hydrological variations are an important factor in the dynamics of aquatic macrophyte growth in tropical wetlands.

Acknowledgements – The authors thank the Environmental Sciences Lab/UENF for the use of their facilities and for logistical support; Faperj for providing research funding; and UENF/Capes for their support through the Masters scholarship program.

(received: Apr 15, 2009; accepted: June 20, 2012)

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