4 DISCUSSION
4.1 Net discharge and erosive
potential
Net discharge (Qnet) from Araguari River towards Amazon
River during the dry season reached 548
m3s-1; it increased to 3,688
m3s-1 during the rainy season.
Therefore, river discharge was the main forcing agent between Araguari
River and Amazon River mouth during the rainy season, which recorded
magnitude 6.7 times greater than that of the dry season.
Urucurituba Channel is the one with the greatest erosive potential, both
in lower Araguari River and in Amazon River mouth (Bailique). Maximum
seasonal Qnet values were recorded in the two points of
the channel (URU1 and URU2), which, consequently, recorded maximum
current velocity values: VURU ≈1.01
ms-1 in the ebb tide and 0.67 ms-1in the flood tide – it is worth emphasizing velocity and depth are
directly proportional to susceptibility to erosion (Novo, 2008). For
example, expressive depths of approximately 28 m in almost the entire
URU1 length, and of approximately 24 m in URU2 were notable (Santos et
al., 2018). There was greater influence of tides spreading from
Urucurituba and Gurijuba-Igarapé Novo channels and from branches in the
floodplain to ARA1, during the dry season. Torres et al. (2018) have
suggested that the recent clogging at Araguari River mouth, due to
drainage of its water by Urucurituba Channel, has contributed to
increase Qss and Qnet at Amazon River
mouth, which has significantly accelerated erosion and accretion
processes in Bailique archipelago region.
Despite the scarcity of information about the region, records show that
mean discharge in June 2011 reached 2,540 m³s-1 (rainy
season) and 316 m³s-1 in December 2011 (dry season) 60
km away from the old river mouth, just downstream the confluence with
Igarapé Tabaco (Santos, 2012). However, Araguari River discharge has
significantly decreased. In June 2013 (rainy season), it decreased to 34
m³s-1 and, subsequently, recorded negative discharge
of –212 m³ s-1 (drought in September 2013), which
indicated permanent flow direction inversion due to greater influence of
the upstream tide, which has fully changed the hydrodynamic pattern of
its mouth (Santos et al. 2018).
Water and sediment transport balances (Figures 4 and 7) have suggested
high hydrological and hydrosedimentometric complexity in the region, as
well as identified accelerated Urucurituba Channel evolution, which not
only resulted in the expansion of a main drainage channel, but induced
severe erosion processes, as well as uncontrolled branching of several
waterbodies and of their connections. For example, Gurijuba-Igarapé Novo
Channel stands out for its connection to Urucurituba Channel, given the
presence of interconnected branches that increase the flow direction
complexity, since these flows interact with each other, are chaotically
distributed across the floodplain and get intensified by the tidal
regime (Gallo and Vinzon 2015; Abreu et al., 2020). However, they remain
intense even during the dry period, when the connection between
floodable fields and the main system still influences the ecological
dynamics in the region (Cunha and Sternberg, 2018).
Several environmental consequences arise from this new hydrological
dynamics since it influences and worsens erosive processes and enables
the development of new channels. This process intensifies the natural
geological fragility of coastal estuarine areas and makes them
apparently more attractive to agriculture and, consequently, to soil
trampling by buffaloes due to riparian vegetation removal (Trimble 1994,
Santos 2006).
4.2 Sediment transport: scenarios and implications
The hydrographic basins of Araguari and Amazonas rivers present
different geological features, which result in white water in Amazon
River and in clear water in Araguari and Igarapé Novo rivers. Thus, the
main physical difference between both water types lies on the amount of
suspended load carried by both drainages, which is even more prominent
during the rainy season (Sioli, 1984), when the clear water in ARA1 and
IGN stations presents SSC < 20
mgL-1 and the white water in other stations present
values > 100 mgL-1 (Figure 5).
Urucurituba Channel recorded concentrations in the range of 130
mgL-1 (URU1) ≤ SSC ≤339
mgL-1 (URU2) in May 2018. Mesotidal intrusion through
URU2 is one of the main forces transporting, and regulating the dynamics
of, suspended solids. Similar situations, although under macrotidal
regime, were observed by Carneiro et al. (2020) in Pará River estuary.
According to Asp et al. (2018), Caeté River estuary presents wide tidal
plains in an extensive range of macrotidal incursions, whose discharge
and elevation lead to seasonal SSS variations. Gensac et
al. (2016) reported impact on Sss dynamics at Amazon
River mouth, mainly during the rainy season, when SSStends to decrease down the estuary, despite the flow peaks observed in
Amazon River during this period (Valério et al, 2018).
In the present case, Qss variations may have happened
due to dilution, sedimentation and resuspension processes (Meade et al.
1985), and it suggests that Araguari River flow in the period of
greatest fluvial discharge (rainy season) overlaps the hydrodynamics of
the tides and substantially decreases the input of suspended sediments
(unconsolidated and subconsolidated). On the other hand, the main factor
controlling this difference in magnitude during the dry period lies on
the dynamics of tides capable of overlapping Araguari River flow, since
they transport white water with high Sss contents from
the Amazon River into the basin, mainly via Urucurituba Channel.
The middle portion of Araguari River does not present high suspended
sediment load because it was formed on Precambrian lands of Guiana
Shield (Allison et al. 1996; Brito 2008; Bárbara et al. 2010; Santos &
Cunha, 2015). However, ARA1 has shown expressive suspended solid
discharge (1,354 t tidal cycle-1) during the rainy
season, which was 3.4 times higher (4,586 t tidal
cycle-1) than that during the dry period (Figure 6).
Thus, in comparison to estimates (Qss = 575 t tidal
cycle-1) by Santos and Cunha (2015), these solid
discharge data suggest greater influence of flood tide currents through
the channels interconnecting both basins.
Therefore, the highest Sss and Qssvalues observed for Urucurituba Channel did not only result from
sediments transported by Amazon River, whose main source derives from
erosive processes in the Andes (Gibbs 1967, Sioli 1984, Meade et al.
1985, Filizola and Guyot 2009), but it is also controlled by the action
of local tidal currents that lead to significant erosion in the
floodplain itself and in its sub-consolidated substrate. Santos et al.
(2018) have observed mean erosion rate of 5 m month-1(60 m year-1) at URU1 site; they also reported that this section of the
channel was only 55 m wide in 2011, but it had significantly increased
to 321 m in 2016. In addition, maximum depth in this section of the
channel was ≈35 m, similar to that of the Northern Channel of Amazon
River (Abreu et al., 2020). If one takes into consideration that erosive
potential increases as depth and flow speed increases (Novo, 2008), it
is possible assuming that Urucurituba Channel tends to maintain and
increase the rates of these processes due to its geomorphological
features. Besides presenting the highest mean depth (28 m) in the
measured cross section (URU1) and 34 m elsewhere (Santos et al. 2018),
this channel also presents the highest mean current velocity among all
channels (≈ 0.89 ms-1 during the dry season and ≈0.85
ms-1 during the rainy season).
The continuous and growing increase in suspended solids transport
confirms the hypothesis about sedimentation evolution and worsening in
the old Araguari River mouth, downstream ARA2 station and Urucurituba
Channel, a process that has strong impact on the region because of the
silting-up process taking place at the old river mouth. Impacts
resulting from this process are significant; among them, one finds loss
of environmental and ecosystem services, such as hydrological regime
regulation, water quality, biodiversity maintenance (Junk et al. 2014),
hydrological connectivity intensification/loss and, mainly, pororoca
extinction (Cunha and Sternberg, 2018). Consequently, new deposition
zones were identified, such as the one at the confluence of Urucurituba
Channel with Araguari River, besides the formation of banks, islands on
the left river bank and the clogging of Araguari channel, just upstream
and downstream the confluence with the great tidal channel. Banks, which
already have vegetation, are part of the new morphodynamics and
depositional system along the river; they may be expanding in response
to sediment transport adjustments observed in this stretch (Wang and Xu
2018). Currents upstream Araguari River present reduced velocity due to
these processes. For example, Urucurituba Channel acts as suspended
sediments’ exporter to floodplains during the dry season. On the other
hand, during spring tides, it imports suspended sediments from
floodplains, from Araguari River fluvial discharge and, finally, from
the reflux discharge coming from Igarapé Tabaco region, during the rainy
season (Figure 7).
Therefore, the hypothesis of significant hydrodynamic and sedimentary
changes was confirmed by intensified erosive events in Bailique
Archipelago, Southern Urucurituba Channel (Figures 4 and 7), which
receives expressive liquid and suspended solids contribution that did
not exist until a few years ago (Torres et al. 2018). This outcome
suggests that Urucurituba Channel indicates negative sedimentary mass
balance (erosion) ranging from 14,675 t tidal cycle-1(12%), during the dry period, to 107,982 t tidal
cycle-1 (88%), during the rainy season (Figures 6 and
7). However, the deposition zone (dry season) lies at Gurijuba River
mouth, which presents intense accretion processes capable of forming
islands, as well as muddy banks that progressively evolve to islands due
to fast vegetation adaptation in this zone (Santana and Silveira 2005,
Torres et al. 2018).
4.3 Water quality variability and tidal propagation influence
According to one of the research hypotheses, seasonal hydrological
effect is one of the main factors influencing water quality dynamics in
this estuarine region (Tables 1 and 3). Increased concentration of
parameters associated with sediments (SSS, Turb, TDS and
Secchi) and with saline intrusion (EC and salt) was also observed by
Santos et al. (2014) in the region in 2011. Such an increase has been
attributed to reduced dilution capacity observed in rivers during the
lowest flow period, as well as to erosion processes taking place in
rivers and channels.
According to Geyer (1995), fresh water coming from Amazon River mouth is
partly maintained by the magnitude of its discharge, which is enough to
produce strong current towards the ocean, even during low flow periods
(Abreu et al., 2020). However, increased salinity level during the dry
season, although incipient, can be attributed to lower river dilution in
this period (hydrological effect of lower water volume and
meteorological effect due to salt “precipitation” through the
atmosphere) – this outcome suggests different oceanic influences on the
Amazonian coast. Salinity level during the rainy season (0.01 ppt)
(Tables 1, 2 and 3) was equal to salt levels observed before the
influence of Urucurituba Channel (Santos et al. 2018). Salinity was a
severe environmental issue in the region between 2013 and 2014, so much
so that Araguari River flow has significantly decreased and enabled
significant saline intrusion in the estuary through the original mouth
(Figure 1) - ≈174 times greater than that of 2011 (ranging from 0.01 to
1.74) (Santos et al. 2018). However, salinity level has significantly
decreased after 2013, due to simultaneous silting and blockage of flow
at the river mouth - salinity value of 0.26 ppt was recorded in March
2015. Later, there was salinity peak reduction, such as the one observed
in lakes connected to Araguari River (Piratuba Lake) - remaining
salinity levels ranged from approximately 0.04 to 0.08 ppt (Cunha and
Sternberg 2018).
The herein observed seasonal DO pattern has suggested opposite effect to
that observed by Barbara et al. (2010) in Cutias do Araguari, 60 km
upstream Gurijuba Channel, which recorded lower DO concentrations during
the dry season (minimum of 6 mgL-1) and higher
concentrations of it during the rainy season (≈8 mgL-1) (Tables 2 and
3). However, DO levels remained higher than 6 mgL-1.
Santos et al. (2014) have found low OD values in lower Araguari River,
downstream the study site (<3.0mgL-1),
during the rainy season. Many of these values also remained low in
subsequent campaigns, after the greatest influence of Urucurituba
Channel (Santos et al. 2018). However, greater oxygen saturation,
upstream Araguari River, may take place due to hydraulic accidents
(dams) (> 7.0mgL-1), where differentiated
turbulence favors atmospheric reaeration processes (Silva et al., 2020).
Araguari River, as well as Gurijuba-Igarapé Novo and Urucurituba
channels, develop in a floodplain, without hydraulic accidents, in
downstream direction, which may lead to reduced reaeration rates (Santos
et al. 2014). On the other hand, Amazon River naturally presents low DO
values very often (lower than 5 mgL-1), mainly due to
high respiration rates, as well as to high particulate and dissolved
organic matter concentrations in water (Ward et al. 2013, 2015,
Sawakuchi et al. 2017). Thus, DO concentration decrease can be easily
attributed to the influence of Amazon River on Araguari River, due to
oxygen demand and nutrient supply such as N, S and P (Ward et al.,
2015). The influence on overall water quality starts at Urucurituba
Channel, which enables higher DO concentrations in IGN and ARA1 stations
(> 6 mgL-1 in the two seasons) and lower
DO concentrations in the remaining monitoring stations (Tables 2 and 3).
However, low oxygen concentration can also be associated with high
turbidity, which reduces light penetration (essential to
photosynthesis), mainly in the region of lakes (Brito 2008; Cunha and
Sternberg, 2018). On the other hand, Damasceno et al. (2015) have found
high DO concentration values (7.18 mgL-1 during the
dry season and 6.52 mgL-1 during the rainy season) in
a section upstream Amazon River mouth (Macapá) - these values were
higher than the ones often observed for these channels.
The observed pH corresponds to acidic water, due to high organic matter
concentration and decomposition by microorganisms (Ward et al., 2013),
which increases during the rainy season. However, Barbara et al. (2010)
observed trend of seasonal pH variation in the middle course of Araguari
River, which recorded very low pH value = 4.7 (confluence of the main
Amapari River tributary) and pH = 5.2 at IGN, which is 54 km away from
the study site. This seasonal pattern was observed in large Amazonian
rivers (Jari River) and in lakes connected to these rivers (Ajuruxi
Lake); besides, it is seen as typical feature of Amazonian waters (Da
Silva et al. 2013, Damasceno et al. 2015, Abreu and Cunha 2017). It
happens because semidiurnal tides (Gallo and Vinzon 2015) take place on
a daily basis in the Amazon River Delta and used to influence almost all
water quality parameters in lower Araguari River until 60-80 km away
from its original mouth, at the time it flowed into the Atlantic Ocean
(Santos et al. 2014). However, nowadays the tidal wave spreads up to 75
km from the new river mouth, as indicated by the negative flow at IGN
station (Figure 4A).
The lower influence of flood and ebb tides on parameters such as DO,
DOsat and SSC differ from that observed
in other studies focused on investigating water quality under tidal
influence in Amazonian estuaries. Moura and Nunes (2016) recorded
significant tidal effects for Turbidity, pH and DO – Turbidity was
mostly evident at low tide, whereas pH and DO were mostly evident at
high tide. Alves et al. (2009) have also found positive correlation
between DO and tidal currents. Araújo (2018) has found positive
relationship between flood tide and parameters such as EC, TDS, salinity
and turbidity in Guajará Bay (Pará), as well as negative correlation to
water temperature and lack of significant effects on pH and DO.
The differentiation caused by tides suggests the interaction between two
water types coming from Araguari and Amazonas rivers. For example, pH
values ranging from 5 to 8 are typical of Amazon rivers with clear water
(Junk et al. 2011). Thus, the smallest spatial variations indicate
stretches presenting intense mix of white water coming from Amazon River
with clear water coming from Araguari River. Both rivers present
slightly higher acidic pH at low tide and values closer to neutral pH at
high tide. On the other hand, increased turbidity, TDS, EC and decreased
water transparency during high tide are highly significant due to
increased transport of suspended load from the Amazon River - these
parameters are highly correlated to QSS.
Differences in tidal amplitudes also affect water quality response and
vary depending on spatial drainage configuration. Tidal amplitude under
syzygy condition is greater downstream the estuary. URU2 and GUR
recorded tidal amplitude in the order of 3 m, whereas URU1 recorded 1.9
m; however, this amplitude decreased to just 1.2 m towards the core of
the hydrographic basin (ARA1) (Figure 1). Thus, the spatial factor
(distance from the Amazon River) reflects on several water quality
parameters and is more intense in Urucurituba Channel than in
Gurijuba-Igarapé Novo Channel. In addition, the morphological drainage
pattern of Gurijuba River tends to hinder water flow from Amazon River
to Araguari River, due to greater distance between the two basins
through this channel. On the other hand, Urucurituba Channel presents
significantly straight drainage channel, hydrodynamic magnitudes
increased by tides and greater influence of water velocity and
discharge, which favor different water quality responses.
A detailed, although summarized, analysis of all water quality and
hydrodynamic variables is available in the supplementary file of this
manuscript (Supplementary Figures 1 and 2).