Introduction
Nitrogen (N) is an essential nutrient for organisms and the excessive
inputs of N have led to serious eutrophication and frequent harmful
algal blooms in aquatic systems (Diaz and Rosenberg, 2008; Di et
al. , 2015). As an important reduced form of N,
NH4+ needs the least energy for the
uptake of phytoplankton and bacteria, and thus commonly shows low water
column concentrations when the primary production is high (Gardneret al. , 2004; Paerl et al. , 2011). Despite within the
conditions of low ambient NH4+concentrations and/or net fluxes (i.e., differences between the
regeneration and uptake of NH4+),
evidence have shown that raid NH4+recycling provides hidden N source for algal species (Bruesewitzet al. , 2015; Jiang et al. , 2019; Gardner et al. ,
2017). Therefore, to comprehensively understand the sources and sinks of
N and serve eutrophication control, it is crucial to evaluate
NH4+ recycling and its contribution to
the ecosystem.
Water column NH4+ recycling involves
the intensively coupling of NH4+regeneration and uptake. The regeneration of
NH4+ is primarily contributed by the
bacterial ammonification of labile organic N and metabolic release of
zooplankton (Gardner et al. , 2004), and the uptake of
NH4+ mainly include planktonic (i.e.,
algae and heterotrophic bacteria) assimilation and bacterial
nitrification (Hampel et al. , 2017). The balance between
NH4+ regeneration and uptake can be
reflected by calculating community biological
NH4+ demand (CBAD), which indicates
net microbial community demand for
NH4+ (Gardner et al. , 2017).
Recycling rates of NH4+ and the CBAD
varied among different water bodies under the influence of various
environmental factors and ecosystem characteristics. Nutrient
concentrations, organic matter, and chlorophyll a (Chl-a )
are common factors influencing NH4+recycling rates in eutrophic lakes, estuaries, and coasts (Bruesewitzet al. , 2015; Gudasz et al. , 2015; Jiang et al. ,
2019). In high turbid rivers, besides the factors mentioned above,
suspended solid (SS) is also an important factor influencing
NH4+ recycling processes (Xue et
al. , 2019). SS supports the metabolism of attached heterotrophic
bacteria (i.e., nitrification, denitrification, and nitrous oxide
emission) by providing ideal oxic or anoxic interfaces with easily
available substrates from SS or its surrounding water (Xia et
al. , 2009; Liu et al. , 2013; Yao et al. , 2016; Zhouet al. , 2019; Zheng et al. , 2017). However, the influence
of SS on microbial NH4+ recycling
remains unclear.
Characterized
by low retention time, drastic flushing effects, and dramatic SS
changes, river-estuary continuums exhibit typical gradients of nutrient
concentrations and dynamics (Mccarthy et al. , 2007b; Bruesewitzet al. , 2015; Liang and Xian, 2018). Previous studies in
river-estuary continuums usually focused on N loads and its exports
(Liang and Xian, 2018), long-term changes of nutrient concentrations
(Liu et al. , 2018), and responses of phytoplankton community to
increased nutrients input (Zhou et al. , 2008). Relatively fewer
studies are focused on NH4+ recycling
in river-estuary continuums and its effect factors.
NH4+ recycling in river-estuary
continuums may not support severe algal blooms as in eutrophic lakes,
while it is still playing an important role in microbial growth and can
impact nutrient dynamics in the systems and even lower coasts.
In recent decades, algal blooms increased dramatically in the estuary of
Yangtze River in terms of frequency, area, and persistence time (Zhouet al. , 2008; Tang et al. , 2006; Dai et al. , 2011;
Jiang et al. , 2014). It has been known that external N inputs
play an important role in supporting algal blooms in the estuary of
Yangtze River (Zhou et al. , 2008), while the contribution of
water column N recycling is unclear. We hypothesized that
NH4+ recycling would provide enormous
N for microbes in the river-estuary continuum of Yangtze River, and the
volume of regenerated NH4+ have the
potential to support algal blooms in the estuary. The aims of this study
are to: (1) quantify NH4+ uptake and
regeneration rates as well as CBAD; (2) examine the effects of SS and
other environmental factors on NH4+recycling rates; (3) evaluate the contribution of regenerated
NH4+ to N budgets and explore
implications. This study is the first to report
NH4+ recycling and CBAD in the
river-estuary continuum of Yangtze River. The results of our study
provide a basis for the comprehensive understanding of N sources in the
Yangtze River and the formulation of N management strategies.