Sedimentary Environment Influences Ecosystem Response
to Nutrient Enrichment
Emily J. Douglas
1
&
Conrad A. Pilditch
1
&
Andrew M. Lohrer
2
&
Candida Savage
3,4
&
Louis A. Schipper
1
&
Simon F. Thrush
5
Received: 7 August 2017 / Revised: 9 May 2018 / Accepted: 11 May 2018
#
Coastal and Estuarine Research Federation 2018
Abstract
As coastal catchment land use intensifies, estuaries receive increased nutrient and sediment loads, resulting in habitats that are
dominated by muddy organic-rich sediments. Increased mud (i.e. silt-clay (particles < 63 μm)) content has been associated with
negative effects on soft sediment biodiversity and ecosystem functioning, but the simultaneous impact of nutrient enrichment on
ecosystem response is unclear. Nutrient recycling and denitrification in estuarine soft sediments represent important ecosystem
functions regenerating nutrients for primary producers and regulating the ability to remove excess terrestrially derived nitrogen. To
test the effect of sedimentary environment on ecosystem resilience to nutrient perturbation, we experimentally enriched sediments
with slow release fertiliser across an intertidal sedimentary gradient (0–24% mud content). The enrichment successfully elevated
pore water ammonium concentrations (median 36 × control) to levels representative of enriched estuaries. Findings show that the
sedimentary environment can influence ecosystem function response to nutrient stress. In particular, denitrification enzyme activity
was suppressed by nutrient enrichment, but the effect was greater as sediment mud content increased. Furthermore, compared with
sandy sediments, sediments with high mud content may restrict nutrient processing (release, uptake or transformation of organic
nutrients by the benthos) facilitating ecosystem shifts toward eutrophication. These results show the value of investigating the
impacts of stressors in different environmental settings and demonstrate that land use practices that increase the proportion of muddy
habitats in estuaries may reduce denitrification which in turn may reduce ecosystem resilience to eutrophication.
Keywords Estuary
.
Denitrification enzyme activity
.
Benthic fluxes
.
Eutrophication
.
Sedimentation
.
Nitrogen
Introduction
Nutrient enrichment and sedimentation are among the primary
stressors for coastal ecosystems globally (Levin et al. 2001).
Estuarine soft sediment ecosystems are often described as ni-
trogen sinks due to their high rates of nitrogen processing and
ability to naturally reduce bio-available nitrogen via denitrifi-
cation (Seitzinger 1988). Denitrification may play a funda-
mental role in ecosystem resilience to the oversupply of nitro-
gen, but its ability to do this may be influenced by the sedi-
mentary environment and potential changes to it. Nitrogen
enrichment and sedimentation often occur in unison during
periods of elevated rain runoff, and while it is clear that the
‘muddying’ (i.e. increases in fine particles < 63 μm) of estu-
aries can negatively affect macrofaunal diversity and ecosys-
tem functions (Pratt et al. 2013; Thrush et al. 2003a, b;
Anderson 2008), it is not known what this means when
compounded with other stressors, particularly increased nutri-
ents. The influence of sedimentary environment on the effects
of nutrient stress in coastal ecosystems has rarely been inves-
tigated in the field (see O'Brien et al. 2009 for an exception)
but such research is needed to better inform management with
respect to setting catchment nutrient limits, for example
(Hewitt et al. 2016;Chapman2016).
Communicated by Marco Bartoli
* Emily J. Douglas
emily.douglas@waikato.ac.nz
1
School of Science, University of Waikato, Private Bag 3105,
Hamilton 3240, New Zealand
2
National Institute of Water and Atmospheric Research, P.O. Box
11-115, Hamilton 3251, New Zealand
3
Department of Marine Science, University of Otago, PO Box 56,
Dunedin, New Zealand
4
Marine Research Institute and Department of Biological Sciences,
University of Cape Town, Rondebosch, South Africa
5
Institute of Marine Science, University of Auckland, Private Bag
92019, Auckland 1142, New Zealand
Estuaries and Coasts
https://doi.org/10.1007/s12237-018-0416-5
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