Nitrogen Cycling in the North Atlantic Ocean and its Watersheds

304 Pages·1996·8.61 MB·English

by  J. N. Galloway, R. W. Howarth (auth.), Robert W. Howarth (eds.)

NITROGEN CYCLING IN THE NORTH ATLANTIC
OCEAN AND ITS WATERSHEDS
NITROGEN CYCLING IN THE NORTH ATLANTIC
OCEAN AND ITS WATERSHEDS
Edited by
ROBERT W. HOWARTH
Reprinted from Biogeochemistry 35(1 )
KLUWER ACADEMIC PUBLISHERS
DORDRECHT / BOSTON / LONDON
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ISBN-13: 978-94-010-7293-9 e-ISBN- 13: 978-94-009-1776-7
DOl: 10.1007/978-94-009-1776-7
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CONTENTS
ROBERT W. HOWARD / Foreword
J.N. GALLOWAY, RW. HOWARTH, A.F. MICHAELS, S.W. NIXON, J.M.
PROSPERO & FJ. DENTENER / Nitrogen and phosphorus budgets of
the North Atlantic Ocean and its watershed 3
J.M. PROSPERO, K. BARRETT, T. CHURCH, F. DENTENER, R.A. DueE, J.N.
GALLOWAY, H. LEVY II, J. MOODY & P. QUINN / Atmospheric
deposition of nutrients to the North Atlantic Basin 27
RW. HOWARTH, G. BILLEN, D. SWANEY, A. TOWNSEND, N. JAWORSKI, K.
LAJTHA, J.A. DOWNING, R. ELMGREN, N. CARACO, T. JORDAN, F.
BERENDSE, J. FRENEY, V. KUDEYAROV, P. MURDOCH & ZHU ZHAO
LIANG / Regional nitrogen budgets and riverine N & P fluxes for
the drainages to the North Atlantic Ocean: Natural and human
influences 75
S.w. NIXON, J.W. AMMERMAN, .. P. ATKINSON, V.M. BEROUNSKY, G.
BILLEN, W.C. BOICOURT, W.R. BOYNTON, T.M. CHURCH, D.M. DITORO,
R ELMGREN, J.H. GARBER, A.E. GIBLIN, RA. JAHNKE, N.J.P. OWENS,
M.E.Q. PILSON & S.P. SEITZINGER / The fate of nitrogen and phos-
phorus at the land-sea margin of the North Atlantic Ocean 141
A.F. MICHAELS, D. OLSON, J.L. SARMIENTO, J.W. AMMERMAN K. FANNING,
R. JAHNKE, A.H. KNAP, F. LIPSCHULTZ & J.M. PROSPERO / Inputs,
losses and transformations of nitrogen and phosphorus in the
pelagic North Atlantic Ocean 181
MICHAEL E.Q. PILSON & SYBIL P. SEITZINGER / Areas of shallow water
in the North Atlantic 227
SYBIL P. SEITZINGER & ANNE E. GIBLIN / Estimating denitrification in
North Atlantic continental shelf sediments 235
FREDRIC LIPSCHULTZ & NICHOLAS J.P. OWENS / An assessment of
nitrogen fixation as a source of nitrogen to the North Atlantic
O~an M1
P.K. QUINN, KJ. BARRETT, FJ. DENTENER, F. LIPSCHULTZ & K.D. SIX /
Estimation of the air/sea exchange of ammonia for the North
Atlantic Basin 275
Biogeochemistry 35: 1,1996.
Foreword
The papers in this special issue of Biogeochemistry summarize the results of
a workshop held on Block Island, Rhode Island, USA, in May 1994. This
was the first of several workshops planned as part of the International SCOPE
Project on Nitrogen Transport and Transformations: A Regional and Global
Analysis. This project is sponsored by SCOPE, the Scientific Committee on
Problems of the Environment, International Council of Scientific Unions,
because of the recognized need to better understand how humans have altered
nitrogen cycling at large regional scales, and what the consequences of this
alteration are.
This first workshop assessed what is known about nitrogen cycling in
the North Atlantic Ocean and its watersheds, with a particular emphasis on
how human activity has altered nitrogen fluxes. Over 40 scientists from 13
nations contributed to the great success of this workshop. The resulting papers
consist of a summary overview, 4 core chapters which reflect the concensus
viewpoints of the 4 working groups of the meeting, and a few notes which
elaborate on particular aspects of nitrogen cycling in the North Atlantic Basin.
I hope that the readers of Biogeochemistry share my enthusiasm and that of
the meeting participants for the many exciting findings presented in this issue.
The Block Island workshop was sponsored by the United Nations Envi
ronment Program, the World Meteorological Organization, and the Mellon
Foundation. The Mellon Foundation also provides additional financial support
for the SCOPE Nitrogen Project. James Galloway and I served as co-chairs
for the meeting. Other members of the organizing committee included Ragnar
Elmgren, Joseph Prospero, Anthony Knap, Anthony Michaels, Jennie Moody,
and Scott Nixon. Scott Nixon also served as local host for the workshop. Sup
port for the meeting was provided by Ellen Yoder of the University of Rhode
Island, Mary Scott Kaiser of the University of Virginia, and Roxanne Marino,
Emily Ehrenfeld, and Dennis Swaney of Cornell University. I am grateful to
all.
Robert W. Howarth
Editor-in-Chief
2
International Scope Project on Nitrogen Transport and
Transformations: A Regional and Global Analysis
Objectives and Activities
• to foster the necessary synergism between scientist of many disciplines
(marine ecologists, forest ecologists, agricultural scientists, microbiol
ogists, atmospheric chemists, oceanographers, hydrologists) in order to
help develop new approaches for the study of nitrogen cycling;
• to refine the global nitrogen budget and develop regional budgets for
selected key and contrasting regions of the world;
• to more fully understand the problems stemming from accelerated nitro
gen cycling, and the inter-relationships among these problems.
Scientific Advisory Committee
Co-chairs:
Robert Howarth (USA)
John Freney (Australia)
Members:
Frank Berendse (The Netherlands)
Pornpimol Chaiwanakupt (Thailand)
Valery Kudeyarov (Russia)
Scott Nixon (USA)
Peter Vitousek (USA)
Zhu Zhao-liang (People's Republic of China)
Consultants:
Ragner Elmgren (Sweden)
James Galloway (USA)
OPE
SCIENTIFIC
COMMITTEE
ON PROBLEMS
OF THE ENVIRONMENT United Nations Environment Programme
Biogeochemistry 35: 3-25, 1996,
© 1996 Kluwer Academic Publishers,
Nitrogen and phosphorus budgets of the North Atlantic
Ocean and its watershed
J, N, GALLOWAyl, R. W, HOWARTH2, A. E MICHAELS3,
S. W. NIXON4, J. M. PROSPER05 & E J. DENTENER6
1E nvironmental Sciences, University ojVirginia, Charlottesville, VA 22903 USA; 2 Ecology
and Systematics, Cornell University, Ithaca, NY 14853 USA; 3 Bermuda Biological Station jor
Research, St. Georges GE-Ol Bermuda; 4Graduate School oj Oceanography, University oj
Rhode island, Naragansett Rl 02882 USA; 5 RSMAS, University ojM iami, Miami, FL 33149
USA; 6 Department ojA irquality, Wageningen, NL 6700 EV Wageningen, The Netherlands
Received 22 March 1996; accepted 22 March 1996
Abstract. Anthropogenic food and energy production extensively mobilize reactive nitrogen
(N) in the watershed of the North Atlantic Ocean (NAO). There is wide spread N distribution
by both hydrologic and atmospheric processes within the watershed of the NAO, resulting
in reactive N accumulation in terrestrial systems. Net denitrification in most estuaries and
continental shelves exceeds the amount of N supplied to the shelves by rivers and requires
a supply of nitrate from the open ocean. Thus riverine N is only transported to the open
ocean in a few areas with the flow from a few major rivers (e.g., Amazon). Atmospheric N
deposition to the open ocean has increased and may increase the productivity of the surface
ocean. In addition, as a consequence of increased Fe deposition to the open ocean (due in part
to anthropogenic processes), the rate of biological N-fixation may have increased resulting in
N accumulation in the ocean. Phosphorus (P) is also mobilized by anthropogenic processes
(primarily food production). Relative to N, more of the P is transported across the shelf to
the open ocean from both estuaries and major rivers. There are several consequences of the
increased availability of Nand P that are unique to each element. However, the control on
primary productivity in both coastal and open ocean ecosystems is dependent on a complex
and poorly understood interaction between Nand P mobilization and availability.
Introduction
Nitrogen (N) is a key element of many biogeochemical processes and can be
a limiting element of aquatic and terrestrial ecosystem processes (Schlesinger
1991; Vitousek & Howarth 1991). However, about 99% of global N exists
as stable atmospheric N2 (Mackenzie et al. 1993) and thus is unavailable
to ecosystems unless it is converted into a reactive N species (reactive N
= NH3, NHt, organic N, NO, N02, HN03, N03, N20S, HN04, HN02,
NO;-, and NO.3). Once created, one species of reactive N can be converted
into other species of reactive N by a variety of chemical and microbial
processes. In addition, reactive N species are very mobile via atmospheric and
hydrologic pathways. After formation, reactive N can only be converted back
to unreactive N2 by denitrification, an anaerobic process which only occurs
4
at significant rates in specific types of ecosystems. Reactive N accumulates
in the environment if the denitrification rate is less than the rate of N -fixation.
In the absence of humans, natural processes create reactive N by biological
N-fixation and lightning. Biological N-fixation occurs in specific microbes
when atmospheric N2 is converted to NH3 by the enzyme nitrogenase. Light
ning produces NO by the reaction of N2 and 02 at high temperatures. The
former process is about two orders of magnitude greater than the latter on a
global basis (Galloway et al' 1995). Human intervention in the N cycle has
increased the formation rate of reactive N by fertilizer production, legume and
rice cultivation, and combustion of fossil fuels. Results from several recent
analyses of the global N cycle (Mackenzie et al. 1993; Ayres et al. 1994;
Galloway et al. 1995) generally agree that anthropogenic activities mobilize
about 10TmoiN yc 1 (Tmol = 1012 moles) and that human activities mobilize
N at rates equal to natural terrestrial processes.
There is significant distribution of anthropogenic N by hydrologic and
atmospheric transport. Combustion of fossil fuels injects reactive N (NO)
directly into the atmosphere. Fertilizer and cultivation N increase the produc
tivity of agricultural landscapes. However, on average, no more than 50%
of the applied N fertilizer is removed by crop harvest; the remainder is lost
to the atmosphere or hydrosphere, or stored in the soil (Howarth et al. this
volume). In addition, N fixed in crops has a short residence time; it is quickly
transformed into human or animal waste, which also results in significant
inputs to the atmosphere and hydrosphere (Howarth et al. this volume). If
the N introduced into the environment by fossil fuel combustion, fertilizer
production and cultivation is not denitrified, then reactive N accumulates in
downwind or downstream ecosystems.
The mobilization, dIstribution and accumulation of anthropogenic N
impacts a number of physical and ecosystem processes. As discussed in
Howarth et al. (this volume), Nixon et al' (this volume), and Michaels et al.
(this volume), increased availability of reactive N increases forest produc
tivity, and as a consequence stores atmospheric CO2, contributes to forest
decline (if soil is N-saturated) and climate change, and results in shifts
in community structure and ecosystem function. It also increases coastal
eutrophication in estuaries (including their wetlands), and results in increased
N supply to oligotrophic mid-ocean gyres with concomitant affects on the
ecology of the upper ocean. In addition to being important to ecosystems,
reactive N also affects atmospheric chemistry (Pro spero et al. this volume).
High levels of NOx (NO + N02) play an important role in the photochemical
production of 03 (Moxim et al. 1994). NH3 is a major source of alkalinity in
the atmosphere and a source of acidity in soils (Schlesinger 1991). Although
N20 is not viewed as a reactive form of N in the troposphere, it adsorbs IR
5
Figure 1. The North Atlantic Ocean and its watershed. The watershed is subdivided into its
drainage basins. The boundary between the shelf and the open ocean is 200m depth.
radiation and acts as a greenhouse gas. In the stratosphere, N20 impacts 0
3
concentrations (Wameck 1988). Thus, any change in the rate of formation of
reactive N (or N20), its global distribution, or its accumulation rate can have
a fundamental impact on many environmental processes.
Our understanding of the global distribution, fate and impacts of anthro
pogenic N is insufficient because of a lack of data. To partially alleviate
this problem, it is necessary to examine the N cycle on a smaller scale
where data are more available, while still keeping the salient features of the
N cycle - natural and anthropogenic sources, atmospheric and hydrologic
transport processes, and agricultural, forested, freshwater, coastal and marine
ecosystems. Hence we focus on a watershed analysis in general, and the
North Atlantic Ocean and its watershed (NAO&W) specifically (Figure 1).
We selected the watershed scale because its physiographic and hydrological
features shape a biogeochemical system; these same features shape systems
of human interaction. We selected the NAO&W because, compared to other
ocean basins, it contains a rich data record of the atmosphere: watershed: ocean
units on N cycling and because of the high degree of N disturbance by human
activities.
Our analysis of the NAO&W N budget was centered about a workshop,
held in May 1994 at Block Island, RI, USA. The central scientific question
of the workshop was:
6
What are the current sources and sinks of nitrogen in the North Atlantic
Ocean and its watershed? How might these have been changed over
natural background levels as a result of human activity?
Since the impact of anthropogenic N on ecosystems is determined in part
by the relative availability of N and P, our investigation of the N budget of
the NAO&W includes an analysis of the magnitude and fate ofP transferred
from continents to the ocean.
To address the question of N cycling on the scale of an entire ocean basin
and its associated watershed, a unique group of individuals assembled, each
with expertise in physical, chemical and biological aspects of marine, coastal,
freshwater and terrestrial ecosystems, the atmosphere, and the connections
among the components. Working groups were formed on the basis of reser
voirs: atmosphere, watershed, coastal and shelf region, and open ocean. Each
working group developed an annual N cycle for their respective reservoir.
The watershed of the NAO and associated coastal segments were further
divided (Figure 1) to investigate spatial variability in N and P fluxes. The task
of each group was to characterize the internal N fluxes and the hydrologic/
atmospheric export losses for their reservoir; input fluxes to the reservoir
were the domain of the upstream or upwind group. To enhance integration,
each group was allowed 'to borrow' members from other groups to address
specific issues of N exchange.
The primary products of the workshop are the working group papers
on the N and P fluxes of each sub-reservoir: atmosphere - deposition to
watershed and oceans (Prospero et al. this volume); watershed - N inputs,
fates, and riverine losses to the coastal margins, and riverine P fluxes to the
coastal ocean (Howarth et al. this volume); coastal and shelf region-fate of
riverine and atmospheric inputs (Nixon et al. this volume); and the open ocean
(Michaels et al. this volume). In addition, in the process of developing these
papers, it became apparent that several topics warranted special attention.
Thus accompanying these papers are four notes: NH3 exchange between the
atmosphere and the NAO (Quinn et al. this volume), N-fixation in the NAO
(Lipschultz and Owens, this volume); denitrification in the ocean boundaries
(Seitzinger and Giblin, this volume); and shelf areas of the North Atlantic
Ocean (Pilson and Seitzinger, this volume).
This paper is a synthesis of the workshop findings. It presents the N and P
budgets for the entire NAO& W system by addressing the following specific
topics:
• Mobilization of reactive N in the NAO& W
• Re-distribution of reactive N among the subsystems of the NAO& W
• Removal of reactive N from the NAO&W relative to sources
• P distribution patterns in the NAO&W