OBRAS Y SERVICIOS PRODUCTO DE CONTRIBUCIONES ECONOMICAS

With an elevated Te Puna Stream discharge in winter, DIN concentrations decreased as the water moved from the upper to lower estuary (Figure 5.3, 5.4). With a 30% increase in Te Puna Stream discharge and DIN input, ammonium concentrations in the upper estuary increased by 28% and the average concentration decreased as water moved into the lower estuary with only a 15% increase relative to the base case. With a 30% increase in stream discharge and DIN (Case 8), the nitrate concentration in the upper estuary increased by 31% and lower estuary by 19%. The DO concentration remained unchanged in the lower estuary with increased stream discharge but in the upper estuary the DO was higher by 0.3 mg/L with the increase in stream and DIN discharge (Case 7, 8).

Despite an increase in stream loading in winter, the downstream DIN concentration was still low which is likely due to phytoplankton uptake and/or denitrification. Phytoplankton uptake in winter, by the fast-growing diatoms can utilize nutrients to reach their maximum growth rate. However, with an increase in stream flow rates and DIN loadings, the overall Te Puna estuarine DIN concentrations did not evoke major changes. This can be related to the low residence times in Te Puna Estuary (Chapter 4), which ranged from 0.5 days in the lower estuary to 1.5 days in the upper estuary. The rapidly flushed system lessens and prevents the accumulation of nutrients due to the high exchange of water between the estuaries and the open boundary.

It is not always the case that increased nutrient levels in freshwater inputs are so quickly flushed from the estuarine system. Although both Te Puna and Waikareao are sub-estuaries within a larger system, Tauranga Harbour, just like some other sub-system within larger estuaries such as the Baltic, the two estuaries are entirely different as in the other sub-system (e.g. the Baltic), the water is poorly flushed (Savchuk and Wulff 2009). For example, through nutrient tracking technique, Neumann (2007) found that nitrogen originating from River Oder remained in the Baltic Sea for a residence times of 30 years due to restricted exchange with the North Sea.

In summer, with reduced stream discharge (Case 9, 10), the overall estuaries‟ nutrient and DO concentrations showed a lateral decline from the upper to lower estuary (Figure 5.5). With reduced Te Puna Stream discharge, the ammonium average concentrations increased by 1% while nitrate and DO concentrations decreased by 3% and 1.7% respectively. The same pattern was observed in Waikareao, where the ammonium concentrations increased by 0.8% while the nitrate and DO concentrations decreased by 5% and 0.5% respectively. The higher concentrations of NH4+ observed in summer under reduced stream flow, indicate

the sensitivity of regeneration to temperature. The reduction in simulated nitrate concentration and increased ammonium concentration in summer under reduced stream discharge suggested that freshwater discharges are the main source of nitrate, while estuarine processes such as sediment fluxes contributed towards ammonium. This source of nutrient is important especially in summer where

spring (Case 11 and 12), the average ammonium concentrations increased slightly by 3% while the nitrate and DO concentration decreased by 0.2% and 2% respectively in Waikareao (Figure 5.6). However, in Te Puna, the average nitrate and DO concentration decreased by 8% and 7% respectively while ammonium increased by 2% in the upper estuary. The low DO in the upper estuary can enhance nutrient release, i.e. ammonium, from the sediment.

Spatial variability observed in Te Puna and Waikareao with different residence times along and across both estuaries in turn can influence the distribution of nutrients. The headland at 1.5 km away from Te Puna Estuary mouth caused increased residence times and consequently a rise in nutrient concentration and lower DO. However in the lower parts of both Te Puna and Waikareao estuaries, rapid tidal exchanges lead to renewal of DO concentrations which can be used for nutrient recycling processes within the estuaries. This spatial variability in residence times was also observed in Mildred Island at Sacramento-San Joaquin Delta where strong north-south gradients in residence times can influence the transport and accumulation of plankton and DO in the southeastern region due to slow tidal mixing but not in the northeastern region due to rapid tidal exchanges with outer channel systems (Lucas et al. 2002; Monsen et al. 2002).

The overall contributions of Te Puna Stream and Kopurereroa Stream towards estuarine DIN concentration are relatively small compared to some major rivers such as the Scheldt, Rhine and Seine which have been linked to nutrient enrichment and phytoplankton blooms in the Belgian Coastal Zone (Lacroix et al. 2007). However model scenarios revealed the sensitivity of such shallow systems with high water exchange with marine waters, to inputs from Te Puna and Kopurereroa streams in the overall estuarine nutrient distribution.

Simulated diatoms appear to bethe dominant phytoplankton group, with marine diatoms in lower estuary and freshwater diatoms in upper estuary, in winter with increased stream discharge (Figure 5.7). This is likely due to reduced water residence times and salinity along with increased nutrient concentrations environment that favored fast-growing phytoplankton such as the diatom (Malone et al. 1988; Harding 1994; Pinckney et al 1999). By contrast in summer with reduced stream discharge, simulated cyanobacteria are the dominant group when

water temperature are warmer and nutrients are lower (Andersson et al. 1994; Piehler et al. 2002) with higher concentrations found in upper Te Puna till the constriction in middle estuary (Figure 5.8).

Figure 5.3 Depth-averaged nitrate (NO3-), ammonium (NH4+) and DO concentrations across Te Puna Estuary for 10% increase in Te Puna Stream flow (A) (Case 5), 30% increase in Te Puna Stream flow (B) (Case 6), 10% increase Te Puna Stream flow and nutrient (C) (Case 7), and 30% increase in Te Puna Stream flow and nutrient (D) (Case 8), in winter.

Figure 5.4 Along-channel average concentrations of ammonium (NH4+), nitrate (NO3-), and DO for Scenarios 7, 8, 9 and 10 for Te Puna Estuary in winter. The channel runs from the upper estuary (0 m) to estuary mouth (2500 m). Results are extracted at 250 m intervals along the main channel. 10% stream: 10% increase in stream discharge; 30%: 30% increase in stream discharge; 10% stream +N: 10% increase in stream discharge and DIN; 30% stream +N: 30% increase in stream discharge and DIN.

Figure 5.5 Depth-averaged nitrate (NO3 -

), ammonium (NH4 +

) and DO concentrations across Te Puna Estuary (A) and Waikareao Estuary (B) in summer with reduced stream discharge of 10% for Te Puna Stream (Case 9) and Kopurereroa Stream (Case 10).

Figure 5.6 Depth-averaged nitrate (NO3-), ammonium (NH4+) and DO concentrations across Te Puna Estuary (A) and Waikareao Estuary (B) in start of spring with increased Te Puna Stream (Case 11) and Kopurereroa Stream (Case 12) nutrient by 10%.

Figure 5.7 Depth-averaged of marine and freshwater phytoplankton concentrations across Te Puna Estuary for 10% increase in Te Puna Stream flow (A) (Case 5), 30% increase in Te Puna Stream flow (B) (Case 6), 10% increase Te Puna Stream flow and DIN (C) (Case 7), and 30% increase in Te Puna Stream flow and DIN (D) (Case 8), in winter. Note the different scales in phytoplankton concentrations.

Figure 5.8 Depth-averaged phytoplankton concentrations for Te Puna Estuary (A) and Waikareao Estuary (B) in summer with reduced stream discharge for Te Puna Stream and Kopurereroa Stream (10%).