The Brantas River is a tropical mid-sized river with mountainous
headwaters, covering a catchment area of around 11,050 km
$^2$
at the east coast of Java, Indonesia. Its watershed is located in one of the
most densely populated regions worldwide, facing alterations by agriculture,
urbanization and aquaculture ponds. Additionally, one of the two major
distributaries of the Brantas River in the lowlands, the Porong River is
affected by inputs from the "LUSI" mud volcano since April 2006. We
investigated spatio-temporal variations in inorganic nutrient biogeochemistry
of the Brantas River, its major distributaries in the lower reaches and its
coastal-estuarine regions and related them to land use and hydrology.
Highest nutrient loadings occurred during the wet periods (November
to April) making up 80% and 87% of the annual dissolved inorganic nitrogen
(DIN: NO
$_3^-$
, NO
$_2^-$
, NH
$_4^
+$
) and phosphorus (PO
$_4^{3-}$
) loads, respectively,
with the Porong River accounting for 90% and 82% of the annual DIN and
PO
$_4^{3-}$
input. During wet periods the estuaries were
flushed with DIN and PO
$_4^{3-}$
rich freshwater, leading to
high concentrations in coastal waters. Much lower nutrient concentrations were
observed in coastal waters during dry periods because of low river discharge
and nutrient load. During dry periods an increased exchange time and increased
biological activity were responsible for estuaries acting as a sink for
NO
$_3^-$
and a source for NH
$_4^ +$
and
PO
$_4^{3-}$
. In contrast, during wet periods most of the
introduced NO
$_3^-$
was directly discharged into coastal
waters without further processing and NH
$_4^ +$
and
PO
$_4^{3-}$
fluxes were slightly lower. Variations in the DIN
composition were mainly related to differences in land use with
NO
$_3^-$
dominating the agriculture-dominated upper Brantas
River and increasing NH
$_4^ +$
and NO
$_2^-$
content in the lower reaches affected by urban wastewater and aquaculture. The
mud volcano affected parts of the Porong River showed drastic changes in the
DIN composition and depletion of dissolved oxygen during low flow periods. In
contrast, during wet periods most of the mud volcano input was diluted by the
large freshwater and inorganic nutrient supply from the upstream regions. Our
results suggest that the densely urbanized Brantas River with multiple
anthropogenic nutrient sources (agriculture, urban sewage release,
aquacultures) leads to an increased export of dissolved inorganic nitrogen and
phosphorus into coastal waters. The enhanced nutrient export supports nutrient
enrichment in coastal waters, can possibly affect the phytoplankton production
and composition, leading to eutrophication within nearshore regions of the
Madura Strait.
