O

Chemically combined nitrogen (N), e.g., nitrate (NO

Marine N-loss to N

NO

Canonical values for the N isotope effect (

Additional information on N-cycling processes can be obtained from the
isotopic composition of NO

It is still under discussion whether the global ocean N budget is in balance.
Current estimates from direct observations and models for N

Ryabenko et al. (2012) also suggested that

The R/V

Station map with satellite data from

NO

NO

A GV Instruments IsoPrime Isotope Ratio Mass Spectrometer (IRMS) coupled to
an on-line He continuous-flow purge and/or trap preparation system was used for
isotope analysis (Sigman et al., 2001; Casciotti et al., 2002; McIlvin and
Altabet, 2005). N

The accumulation of biogenic N

Isotope effects are estimated using the Rayleigh equations describing the
change in isotope ratio as a function of fraction of remaining substrate.
The following equations are used for a closed system (Mariotti et al.,
1981):

The overall isotope effect for N-loss can also be estimated from the

For all equations, the slope represents

Since the closed system equations assume no loss or resupply of substrate or
production in a water parcel, they are appropriate where there is little
mixing and/or advection is dominant over mixing. The open system equations
take into account supply from or loss to surrounding water parcels, e.g.
mixing dominance. Both cases represent extreme situations. In the next
section, we will estimate and compare

To do so, we need to estimate the fraction of NO

However, the assumption of Redfield stoichiometry may not be appropriate in
this shallow environment due to preferential release of PO

A third way to estimate

Temperature vs. salinity plots. In

During the study period, there was active coastal upwelling especially at
station 63 as seen by relatively low satellite sea surface temperatures,
higher chlorophyll

As a consequence of active upwelling sourced from the offshore ODZ, the
oxycline was very shallow at our in-shore stations. O

O

Both Si(OH)

In contrast to other nutrients, NO

Transects off the Peru coast for

As a consequence of kinetic isotope fractionation during N-loss, the N and O
isotope composition of NO

As expected for NO

Relationships between

The difference in

N deficits, biogenic N

The

Isotope effects were calculated using Eqs. (1) to (6) to compare closed
vs. open system assumptions as well as different approaches to estimating

NO

Raleigh relationships used to estimate

N deficit, biogenic N in N

To assess the influence of the various N cycle processes that have
NO

Nitrite oxidation has its own unique set of isotope effects (Casciotti, 2009;
Buchwald and Casciotti, 2010). Nitrite oxidation incurs an unusual inverse N
isotope effect varying from

Past studies have found NO

We observed, for the first time, a significant linear relationship for
NO

Cross-plots of biogenic N in N

In principal, we can estimate NO

NO

The inverse kinetic isotope effect associated with NO

Given that

The deviations from a 1 : 1 relationship for NO

As described above, the Rayleigh fractionation equations (Eqs. 1 to 6) are
used here to estimate

Linear regression coefficients for

However,

The different approaches for estimating the

Estimates of

Closed system estimates of

There are several reasonable explanations for these relatively low

The low

The inshore Peru ODZ is distinguished from offshore by its high productivity
as a consequence of coastal upwelling as well as possible greater influence
from benthic processes. To examine impact on N-loss processes and their
isotope effects, we investigated the dynamics of N and O isotope of
NO

We found that N-loss representing the net effect of partial denitrification,
anammox, and nitrification produced, in sum, large variations in isotopic
composition. NO

We observed, for the first time, a positive linear relationship between
NO

The difference in

A number of different approaches for estimating

Our results further support geographical variations in the

Data for this paper are available on the Data Management Portal for Kiel
Marine Sciences hosted at GEOMAR: