Sobre la situación económica de las mujeres en la región Ayacucho

Overview

Diel changes in dissolved oxygen were used to examine gross primary production, respiration and net ecosystem metabolism within Te Puna and Waikareao estuaries. For each of the four sampling periods, gross primary production exceeded respiration demonstrating that both sites were net autotrophic.

Introduction

Estuaries are transition zones between land and sea through which land-derived nutrients reach the coastal areas. Increased nutrient loads to estuaries have resulted in increased nuisance and toxic algal blooms and degradation of water quality (Howarth et al. 2002; Paerl et al. 2006). One way to assess how a coastal ecosystem will respond to increased loads of nutrients is to make an integrative measure of its ecological properties. The condition can be determined by estimating the net ecosystem metabolism (NEM), which represents the overall metabolic balance of an ecosystem (Howarth et al. 1996). Once the nutrients and organic matter are in the estuary, transformations occur as a result of biogeochemical cycling, e.g. primary production and respiration processes. These transformations are important because the combination of net fluxes of water, dissolved nutrients and organic matter will determine the role of the system as autotrophic (production exceeds respiration) or heterotrophic (respiration exceeds production).

Freshwater inflows into estuaries are changing in most estuaries because of changes in land use and cover, water diversion for human uses and climate effects. These changes generally result in changes in timing of pulse events, increase and in some cases decrease in freshwater inflows. Freshwater inflow transports sediment, nutrients and organic matter from a catchment to an estuary. Thus the inherent variability in freshwater inflow affects nutrient and organic loading which in turn can be linked to estuarine metabolic rates (D‟Avanzo et al. 1996; Caffrey 2004). Spatial and temporal variability in other environmental conditions may also modify estuarine ecosystem metabolic rates. Estuarine metabolic rate can be affected by the interaction of nutrient and organic matter loading with light

of an estuary (Russell and Montagna 2007). Increased loading of nutrients leads to increased NEM by stimulation of production over respiration. Changes in NEM may be driven by environmental conditions that vary temporally on daily scales such as rain and inflows to seasonal scales such as temperature.

Within an aquatic system, metabolic rates vary temporally and spatially. For example in San Francisco Bay, seasonal phytoplankton blooms shifted the system from heterotrophy to autotrophy (Caffrey et al. 1998). In some estuaries, variation along salinity and depth gradients has also been observed, where NEM changes from heterotrophy to autotrophy along the estuarine gradient (Howarth et al. 1996; Raymond et al. 2000). Most NEM studies have focused on large estuarine systems, whereas both Te Puna and Waikareao estuaries are shallow tidally dominated estuaries. The objective of this appendix is to present the results of net ecosystem metabolism (NEM) from diel changes in dissolved oxygen measurements over four sampling periods (summer, winter, start of spring and end of spring) in both Te Puna and Waikareao estuaries.

Methods

Consecutive hourly dawn-dusk-dawn measurements of dissolved oxygen taken by CTD were used to estimate each estuary‟s rates of gross primary production (GPP), respiration (RESP), and net ecosystem metabolism (NEM) in mg O2 L-1 d-1

based on methods of Wiegner et al. (2003). For location of the sampling sites, refer to Chapter 2, Figure 2.1. Rates of the whole system net primary production (NPP) were first calculated from the dissolved oxygen concentration for the first dawn of the 24 h sampling period (dawn1) to dusk:

NPP = (O2dusk – O2dawn1)/t +AS

where O2 = dissolved oxygen concentration measured (mg L-1), t = the number of

h between the measurements and, AS = the diffusive flux (g m-2 h-1) across the air- water interface. RESP was calculated from the decrease in the dissolved oxygen concentration from dusk to dawn2 (the second dawn of the 24 hour sampling):

RESP = (O2dusk – O2dawn2)/t +AS

GPP was estimated from the sum of NPP and RESP, and NEM as the difference

over a diurnal cycle (i.e. RESPday = RESPnight; Wiegner et al. 2003). Oxygen

diffusion across the air-water interface was calculated as:

d t

k

S

AS





where S is the mean fractional saturation deficit (%), DO is the percent oxygen saturation over the time interval dt (in this case an hour). The coefficient, ks, was

assumed to be 0.5 g O2 m-2 hr-1 at zero DO concentration (Caffrey 2004).

Results and discussion

NEM was positive indicating that both Waikareao and Te Puna were autotrophic

(Figure A2.1). The GPP and GRP rates at the start of spring were 1.2-1.4 times higher than rates in other seasons. The NEM rates at both Waikareao and Te Puna were remarkably consistent through the four sampling periods. Apart from the higher values in early spring, NEM showed little seasonal variation in both estuaries.

Large seasonal variations in estuarine metabolic rate were not evident in this study, unlike patterns in other estuaries examples as reported by Caffrey (2004) and Russell and Montagna (2007). A direct consequence of the net export of nutrients from these two estuaries is that there may be ample supplies of nutrients to allow photoautotrophic behaviour regardless of season and may support production in other parts of the harbour. The NEM showed that both estuaries were net autotrophic during the four sampling periods while, previous studies in other estuarine environments have shown that the NEM switches between net autotrophic and net heterotrophic seasonally (Howarth et al. 1996; Caffrey 2004; Russell and Montagna 2007). The net autotrophy in Waikareao and Te Puna could be due to the shallow environment where benthic production likely plays a greater role than in deeper estuaries where many previous studies on estuaries were focused (Caffrey et al. 1998; Ram et al. 2003). Changes in catchment activities, hydrology and organic or nutrient enrichment along with influences of environmental factors such as temperature and light can be influential on estuarine metabolic activities as once in the estuaries, biogeochemical cycling takes place

which can lead to a balance between autotrophy and heterotrophy in estuaries (Caffrey 2004; Russell et al. 2006; Russell and Montagna 2007).

Figure A2.1 Seasonal variations in gross production (GPP), gross respiration (GRP) and net ecosystem metabolism (NEM) in Waikareao (A) and Te Puna (B) estuaries. W: winter; SS: start of spring; SE: end of spring; S: summer.