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Eutrophication is a serious threat to aquatic ecosystems globally with pronounced negative effects in the Baltic and other semi-enclosed estuaries and regional seas, where algal growth associated with excess nutrients causes widespread oxygen free "dead zones" and other threats to sustainability. Decades of policy initiatives to reduce external (land-based and atmospheric) nutrient loads have so far failed to control Baltic Sea eutrophication, which is compounded by significant internal release of legacy phosphorus (P) and biological nitrogen (N) fixation. Farming and harvesting of the native mussel species (Mytilus edulis/trossulus) is a promising internal measure for eutrophication control in the brackish Baltic Sea. Mussels from the more saline outer Baltic had higher N and P content than those from either the inner or central Baltic. Despite their relatively low nutrient content, harvesting farmed mussels from the central Baltic can be a cost-effective complement to land-based measures needed to reach eutrophication status targets and is an important contributor to circularity. Cost effectiveness of nutrient removal is more dependent on farm type than mussel nutrient content, suggesting the need for additional development of farm technology. Furthermore, current regulations are not sufficiently conducive to implementation of internal measures, and may constitute a bottleneck for reaching eutrophication status targets in the Baltic Sea and elsewhere.
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Cleaning up seas using blue growth initiatives: Mussel farming for
eutrophication control in the Baltic Sea
Jonne Kotta
, Ants Kaasik
, Kiran Liversage
, Merli Rätsep
, Francisco R. Barboza
Lena Bergström
, Per Bergström
, Ivo Bobsien
, Kristjan Herkül
, Per R. Jonsson
Samuli Korpinen
, Patrik Kraufvelin
, Peter Krost
, Odd Lindahl
, Mats Lindegarth
Maren Moltke Lyngsgaard
, Martina Mühl
, Antonia Nyström Sandman
Henrik Skov
, Jouko Rissanen
, Andrius Šiaulys
, Aleksandar Vidakovic
, Elina Virtanen
Estonian Marine Institute, University of Tartu, Mäealuse 14, EE-12618 Tallinn, Estonia
Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Box 7050, SE-75007 Uppsala, Sweden
GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, DE-24105 Kiel, Germany
Department of Aquatic Resources, Swedish University of Agricultural Sciences, Skolgatan 6, SE-74242 Öregrund, Sweden
Department of Marine Sciences Tjärnö Marine Laboratory, University of Gothenburg, Tjärnö, SE-45296 Strömstad, Sweden
Novia University of Applied Sciences, Raseborgsvägen 9, 10600 Ekenäs, Finland
Marine Research Centre, Finnish Environment Institute,FIN-00790Helsinki, Finland
Novia University of Applied Sciences, Raseborgsvägen 9, 10600 Ekenäs, Finland
Coastal Research and Management,Tiessenkai12, D-24159 Kiel, Germany
Musselfeed AB, Hallgrens väg 3, SE-47431 Ellös, Sweden
Orbicon, Department for Nature and Environment, Jens Juuls vej16, 8260 VibyJ., Denmark
AquaBiota Water Research, Löjtnantsgatan 25, SE-11550 Stockholm, Sweden
Sankt-Petersburg Research Centre of Russian Academy of Science, University embankment 5, 199034 St.-Petersburg, Russia
DHI, Agern Alle 5, 2970 Hørsholm, Denmark
Marine Research Institute, Klaipeda University, Universiteto ave. 17, LT-92294 Klaipėda, Lithuania
Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, Box 7024, SE-75007 Uppsala, Sweden
Environmental and Marine Biology, Åbo Akademi University, Finland
Mussel farming is a viable internal mea-
sure to address Baltic Sea eutrophica-
Rates of nutrient removal depend on sa-
linity at the regional scale and food
availability at the local scale.
Cost effectiveness of nutrient removal
by mussel farming depends also on
farm type.
Total farm area needed for achieving
HELCOM nutrient reduction targets is
abstractarticle info
Science of the Total Environment 709 (2020) 136144
Corresponding author.
E-mail address: (J. Kotta).
Present address: Department of Environmental Sciences, University of Helsinki, Finland.
Present address: Department of Aquatic Resources, Institute of Coastal Research, Swedish University of Agricultural Sciences, Öregrund, Sweden.
0048-9697/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage:
Article history:
Received 18 October 2019
Received in revised form 13 December 2019
Accepted 14 December 2019
Available online 19 December 2019
Editor: Ashantha Goonetilleke
Blue growth
Eutrophication control
Internal measures
Mussel farming
Baltic Sea
Eutrophication is a serious threat to aquatic ecosystems globally with pronounced negative effects in the Baltic
and other semi-enclosed estuaries and regional seas, where algal growth associated with excess nutrients causes
widespread oxygen free dead zonesand other threats to sustainability. Decades of policy initiatives to reduce
external (land-based and atmospheric) nutrient loads have so far failed to control Baltic Sea eutrophication,
which is compounded by signicant internal release of legacy phosphorus (P) and biological nitrogen
(N) xation.Farming and harvestingof the native mussel species(Mytilus edulis/trossulus)is a promising internal
measure for eutrophication control in the brackish Baltic Sea. Mussels from the more saline outer Baltic had
higher N and P content than those fromeither the inner or centralBaltic. Despite theirrelatively low nutrient con-
tent, harvesting farmed mussels from the central Baltic can be a cost-effective complementto land-based mea-
sures needed to reach eutrophication status targets and is an important contributor to circularity. Cost
effectiveness of nutrient removal is more dependent on farm type than mussel nutrient content, suggesting
the need for additional development of farm technology. Furthermore, current regulations are not sufciently
conducive to implementation of internal measures, and may constitute a bottleneck for reaching eutrophication
status targets in the Baltic Sea and elsewhere.
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
1. Introduction
Eutrophication is a global threat to many aquatic ecosystems and its
negativeeffects are particularly pronounced in semi-enclosed estuaries
and regional seas (Diaz and Rosenberg, 1995;Conley et al., 2009a;
Rabalais et al., 2009). Excessive amounts of nitrogen (N) and phospho-
rus (P) from present-day and legacy sources support massive algal
blooms which results in widespread and increasing oxygen free dead
zones(Breitburg et al., 2018), increasing susceptibility to ocean acidi-
cation (Cai et al., 2011), reduced biodiversity and loss of ecosystem
functions and services (Smith, 2003;Riedel et al., 2016). In the Baltic
Sea, a multi-jurisdictional water body, N40 years of international efforts
to reduce external nutrient (N and P) inputs have failed to solve the eu-
trophication problem (Fleming-Lehtinen et al., 2015;Andersen et al.,
2017). Today, 97% of the marine area is considered as degraded due to
eutrophication (HELCOM, 2018,Fig. 1) and despite signicant reduction
in external loads, the total P pool in Baltic Sea waters continues to in-
crease (Savchuk, 2018) and internationally agreed upon water quality
targets are still not met. To date, management actions have primarily fo-
cused on minimizing external loads, i.e., terrestrial point sources and
diffuse nutrient inputs. Agriculture is targeted in many cases (Larsson
and Granstedt, 2010) but the internal loads of legacy P released from
marine sediments (Vahtera et al., 2007;Conley et al., 2009a) and atmo-
spherically xed N are often neglected (Vahtera et al., 2007), as are non-
food nutrient sources (Hamilton et al., 2018).
Aquaculture is a key component of the EU Blue Growth strategy (EC,
2012) and can have signicant positive and negative effects on water
quality. Aquaculture is the fastest growing food-producing sector and
currently represents nearly 50% of global sh, crustacean and mollusc
production (FAO, 2018). Marine bivalves, e.g., mussels, oysters, clams
and other shellsh, are often referred to as extractive species as these
lter feeding species act as nutrient sinks by ingesting particles
suspended in the water column. Importantly, harvesting of cultivated
mussels removes both N and P, thereby improving water quality in af-
fected areas (Carlsson et al., 2012;Kraufvelin and Díaz, 2015).
Aquaculture can also make a positive contribution to circularity and
nutrientrecycling. Most internal eutrophication control measuresmake
P unavailable for re-use through, e.g., bottom water oxygenation to
change sediment redox status and the binding of P to iron
(Stigebrandt et al., 2015) or aluminium treatment to effectively immo-
bilize P in the sediment (Rydin et al., 2017). Unlike the aforementioned
measures, harvesting of internally produced biomass, (i.e. farmed mus-
sels) offers the potential for efcient recirculation of nutrients from sea
to land. Harvested mussels can be used to produce feed for chickens
(McLaughlan et al., 2014)orsh (Vidakovic et al., 2015), as well as for
human consumption (Gren et al., 2009). Harvested mussels can also
be used for bioenergy production (Hu et al., 2011;Nkemka and Murto,
2013), or as a soil amendment.
The failure to control Baltic Seaeutrophication through external nu-
trient loadreduction measures has highlighted the needfor in-situ (in-
ternal) methods to lower nutrient concentrations in the water column,
e.g. through geoengineering (Stigebrandt et al., 2015;Rydin et al., 2017)
or biomass harvesting (Gren et al., 2009). Intensive shing of commer-
cial or non-commercial shes (e.g., three-spined stickleback, round
goby) has been proposed as an alternative means for removing nutri-
ents from theBaltic Sea. However, this could have unknown and poten-
tially catastrophic consequences for marine biodiversity due to the role
as top or intermediate predators that these species have in littoral hab-
itats. While internal measures for nutrient regulation are not a universal
means of controlling eutrophication, they should be considered when
feasible external measures have been tried and found to be inadequate
(Savchuk, 2018). It should be noted, however, that many internal mea-
sures havebeen associated with high costs for nutrient removal (Lurling
et al., 2016) aswell as undesirable secondary effects such as damage to
benthic habitats (Stadmark and Conley, 2011), potentially harmful
shifts in thermal regime (Conley, 2012) and/or food web impacts
(Naylor et al., 2001). The Baltic Sea region is an important test case
highlighting the opportunities and challenges of farming native bivalve
species as an internal measure to mitigate the adverse effects of coastal
eutrophication. The region has a long, well-documented history of eco-
system deterioration, high data density and multiple cross-border envi-
ronmental management actions to counter marine eutrophication
(Reusch et al., 2018).
Farming of the ubiquitous blue mussel species complex (Mytilus
edulis/trossulus,Stuckas et al., 2009) has been proposed as an internal
measure for eutrophication control in the brackish Baltic Sea (Lindahl
et al., 2005;Gren et al., 2009;Petersen et al., 2014;Schröder et al.,
2014;Ozoliņa, 2017;Kiessling et al., 2019). Mussel farming has also
been criticized as being not cost effective (Hedberg et al., 2018)and
harmful to the environment (Stadmark and Conley, 2011).
Blue mussels are marine species and form hybrid zones within the
Baltic Sea (Stuckas et al., 2009). While individuals are able to survive
down to salinities of 45 practical salinity units (PSU), they grow better
in high salinity conditions where they do not need to expend as much
energy on osmoregulation (Maar et al., 2015). Blue mussels are primary
consumers and usually the dominant species (i.e. main contributor to
abundance and biomass) in the environments where they occur, and
consequently their sustainable harvest is not expected to produce cas-
cading effects or other impacts on the stability of the food web.
Blue mussel farming relies on recruitment of free-swimming larvae
(veligers) from wild populations that are entrained into the water col-
umn and passively dispersed from natural mussel reefs. After dispersal,
2J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
veligers attach themselves to available substrates, including objects in
the water column, e.g., mussel farms. Thus, determining how to best al-
locate areas suitable for mussel farming requires consideration of the
connectivity between candidate farm sites and natural mussel reefs in
order to dene areas that do not require articial mussel seeding.
Farms usingropes with high surface area perunit length, i.e. ribbons,
Swedish bands or so-called fuzzy ropespromote higher rates of larval
settlement. After settlement, it is necessary to account for the way in
which salinity, food availability and wave action affect growth rates
and to select sites with the highest harvest potential. A typical Baltic
Sea mussel farm has an area of a b5 ha and consists of 10100 km of
rope suspended at different depths (Holmer et al., 2015;Kraufvelin
and Díaz, 2015). Cost effectiveness of the farms is dependent on nutrient
and salinity levels as well as the type of equipment for culturing mus-
sels, with specialized ropes that optimize veliger recruitment being
the most effective for culturing the small mussels found in the Baltic.
Harvest rates are usually expressed in units of mass of mussels per
metre of rope. Mussels are harvested one to two years after recruitment,
depending on site productivity. As farmed mussels spend their entire
life suspended in the water column, they are less affected by contami-
nated sediments than benthic dwelling organisms but can be suscepti-
ble to contamination by algal toxins (Sipiä et al., 2001).
A synthesis of a large number of recent measurements of farmed
mussel growth in the Baltic Sea and a new model chain for predicting
growth and nutrient removal potential across key environmental gradi-
ents are presented. The relationship between wild mussel production
and predicted nutrient removal through harvest of farmed mussels
was quantied by modelling occurrence of wild mussels throughout
the whole Baltic Sea.A biophysical dispersal model wasused to analyse
direction and distance of larval drift from each natural mussel reef. Next,
spatially explicit and empirically modelled growth rates of farmed mus-
sels were combined with measured N and P concentrations in mussels
Fig. 1. Areas of the Baltic Sea (coloured; 97% of its total surface area) that currently have unacceptable water quality with respect to eutrophication. Different colours indicate different
water quality classes. The 2013 HELCOM Ministerial Meeting agreed on the amount of reduction in emissions for nitrogen (N) and phosphorus (P) in different sub-basins of the Baltic
Sea in order to meet goals of the Baltic Sea Action Plan. However, trend-based estimates demonstrate that the maximum allowable nutrient inputs are still exceeded in the Central and
Inner Baltic Sea. The excesses are shown as numbers in the different sub-basins. Blue hatched areas show the best mussel growth location separately for Outer, Central and Inner
regions predicted by the model. Within these regions, blue rectangles show the surface area of future mussel farms that are needed to meet the basin-specic goals of nutrient load
reduction dened in the Baltic Sea Action Plan. (For interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article.)
3J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
harvested from production-scale farms to quantify nutrientremoval po-
tential. Finally, farm-scale nutrient removal estimates were upscaled to
predict thetotal area of musselfarms needed to make a meaningfulcon-
tribution to reducing Baltic Sea eutrophication.
2. Material and methods
2.1. Study area
The Baltic Sea is shallow, brackish and has almost no tide but expe-
riences intense seasonality in temperature and inow. It is heavily af-
fected by eutrophication with N and P concentrations showing
decreasing and increasing trends, respectively (HELCOM, 2018;
Savchuk, 2018).
For most of the analyses presented here, the Baltic Sea was divided
into Outer (Kattegat and Belt Sea), Central (Northern Baltic Proper,
Western and Eastern Gotland Basins, Gdansk Basin and Bornholm
Basin) and Inner regions (Bothnian Bay, Bothnian Sea, Archipelago
Sea, Åland Sea, Gulf of Finland andGulf of Riga), representing the gradi-
ent from the near-oceanic (Outer) to brackish-water conditions (Fig. 2).
Despite salinity constraints, several characteristics of the Baltic Sea
favour mussel farming for eutrophication control. First, the Baltic Sea
is very eutrophic and food is only rarely a limiting factor for mussels
(Kotta et al., 2015). Thus, within suitable habitat ranges, elevated re-
source availability can compensate for growth limitation associated
with reduced salinity (Kotta et al., 2015). Second, high nutrient concen-
trations in the water require in-situ removal actions for which mussel
farming is promising. Finally, more than forty years of international
agreements and land-based measures have failed to solve the problem
of Baltic Sea eutrophication; large amounts of money have been allo-
cated to reduce inputs from land with variable, often minimal, effects
(Helin, 2013).
2.2. Mussel farms
Harvest data are reported from three farms, one each in the Outer,
Central and Inner Baltic (Table 1 and Fig. 1). The Kumlinge farm (Inner
Baltic) is located in the Åland archipelago. It was established in spring
2010 and harvested in November 2012. The farm technology consisted
of four 120 × 3 m nets with a mesh size of 15 cm fastened to oating
Fig. 2. Location of samplingpoints for the distribution and growth of blue mussels. Colours depict different sub-regions of the Baltic Sea.(For interpretation of the references to colour in
this gure legend, the reader is referred to the web version of this article.)
4J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
plastic pipes. The average water depth was 8 m and average bottom
water current speeds were 34cms
(Kraufvelin and Díaz, 2015).
The Sankt Anna farm (Central Baltic) is located on a sheltered site in
the Swedish Östergötland archipelago. The farm was established in
spring 2016 and harvested in October 2018. The farm technology
consisted of spat catchingrope developed by Quality Equipment Ltd.
and optimised for settling of small mussels. Ropes were hung at a
depth of 212 m. Average water depth at the site was 20 m and bottom
water current speeds were low. The Kiel farm (Outer Baltic) is located
near Kiel,Germany. The farm technology consists of ropes with collector
bands and socks optimised for the production of large mussels. Ropes
were suspended at depths of 13.5 m and average water depths ranged
between 7 and 11 m (Schröder et al., 2014). The farm was established in
2011 and the rst harvest took place in September 2012.
2.3. Spatial mapping
The blue mussel distribution data were combined from different
sources: benthos database of the Estonian Marine Institute, University
of Tartu (; the
VELMU database, Finnish Environment Institute (http://www.
ymparisto./en-US/VELMU); the database of the Swedish National
Monitoring Programme (, benthic inventory data
collected by AquaBiota (
inventories-using-underwater-video/), the database of Marine Re-
search Institute, Klaipeda University; EurOBIS (http://www.eurobis.
org/) and EMODnet (
(Fig. 2). Altogether, data from 226,031 stations from coastal hard and
soft bottom habitats of the Baltic Sea were included in this study. This
dataset was based on a regional sampling and sample processing proto-
col developed for the HELCOM COMBINE programme (HELCOM, 2015).
The stations included were sampled at least once in summer (June to
August) between 2005 and 2015. On hard bottoms, blue mussels were
collected by divers using a standard bottom frame (0.04 m
) and/or a
hand-held drop camera operated from small motorboats with recording
devices operated on the surface. On soft bottoms, samples were col-
lected using different benthos grabs (sampling area 0.020.1 m
Quantitative samples were sieved in the eld using 0.25 mm mesh
screens. The residues were stored at 20 °C and subsequent sorting,
counting, weighing and measuring of blue mussels were performed in
the laboratory.
Oxygen measurements under the farms were made with a JFE
Advantech optical DO sensor (
The majority of existing experimental measurements of mussel
growth in the Baltic Sea (n= 14,944) were used to model the potential
growth and yields across the key environmental gradients. This includes
the original data of the INTERREG Baltic EcoMussel and Baltic Blue
Growth projects as well as data from different national research initia-
tives from Estonia, Finland, Sweden, Denmark and Germany (Fig. 2).
2.4. Mussel tissue analysis
Nutrientcontent was analysedfor 124 samples of blue mussel tissue.
In each case, 100150 g of fresh material (shells, soft tissue and associ-
ated water) were analysed in the following manner. Whole frozen mus-
sels were removed from the freezer and thawed. A portion of the
thawed mussels (shells, soft tissues and associated water) were manu-
ally crushed using a mortar and pestle. Between 100 and 150 g of the
crushed mussels were weighed. This weight is reported as the sample
wet weight. The samples were then freeze-dried at 80 °C and
weighed. They were then oven dried at 105 °C (to remove any residual
moisture) and weighed again to determine dry mass and dry matter
Prior to the nutrient analysis, dried material was ltered through a
1 mm sieve. Total N measurements were performed by the laboratories
of Swedish University of Agricultural Sciences using the total Kjeldahl
nitrogen (TKN) method. Total phosphorus (P) concentrations were
analysed by Agrilab AB. Samples were acidied using sulfuric acid. P
concentrations were obtained using ICP-AES.
2.5. Statistical analysis of nutrient concentrations
In order to account for regional variability in the nutrient content of
mussels, samples were classied into those obtained in the Outer, Cen-
tral or Inner Baltic Sea. These three functionally different regions were
used to account for the regional-specic nutrient accumulation in mus-
sels when assessing the potential of nutrient removal through
Because the sampling design was unbalanced, i.e., the same number
of samples were not available for the different months across regions,
only the samples collected from the Outer Baltic were used to dene
the best way of grouping the samples obtained in the different seasons
for subsequent analyses. A Tukey's Honest Signicant Difference (HSD)
post-hoc test of an ANOVA predicting wet weight P concentrations as a
function of month indicated that samples from March, April, May and
June belonged to the same group (spring) and had no overlap with
the group of samples collected in other months (other). This grouping
was corroborated by the analysis of wet weight N concentrations.
To facilitate the comparison with mussel harvest values, which are
typically reported as total mass of mussels (i.e. shells, soft tissue and as-
sociated water), ANOVA analyses were performed on wet weight con-
centrations. Pairwise differences were assessed using the Tukey's HSD
test. The ANOVAs tested for the xed effects of region (Outer, Central
and Inner), season (spring or other) and their interaction.
2.6. Modelled environmental variables
Care was taken to select the most relevant ecological variables in
order to reachthe most robust predictions about the role of the environ-
ment for blue mussel occurrence and growth. When the variable selec-
tion is inadequate, a model may include irrelevant variables and its
predictive power is low (MacNally, 2000). Earlier studies have shown
Table 1
Summary of environmental conditions, nutrient removal and economic factors for three
blue mussel farms in the Baltic Sea.
Variable Unit Kumlinge Sankt Anna Kiel
Latitude 60.2147°
58.3564° N 54.3755° N
Longitude 20.7524°
16.9368° E 10.1634° E
Region of the Baltic
Inner Central Outer
Average salinity 6 6 15
Chlorophyll a
mg m
2.0 2.0/3.5/1.0 2.3/4.5/0.9
Farm technology Nets Spat
Ropes with
collector bands
Nitrogen removal at
kg ha
83 140 148
3.74 23.26 22.25
Phosphorus removal
at harvest
kg ha
6.4 10.8 10.8
0.29 1.80 1.63
Farm size ha 0.90 4.0 0.30
Long line length km 20 24 2
Long line density km ha
22.2 6.0 6.7
Observed biomass
tonnes 14.4 81.50 5.00
kg m
0.72 3.40 2.50
Modelled biomass
kg m
0.75 1.3 14.7
Investments kg
5.45 0.35 0.36
Operational expenses kg
3.08 0.17 1.49
Total costs kg
8.52 0.52 1.85
N removal cost kg
1638 76 208
P removal cost kg
21,300 981 2846
5J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
that water salinity, temperature conditions, and food availability (a
product of phytoplankton concentration and water ow) mostly
shape the distribution and growth of blue mussels at the Baltic Sea
scale (Kotta et al., 2015).
Model inputs for the physical and biogeochemical conditions in the
Baltic Sea were obtained from the products
open access data portal (
portfolio/access-to-products/). These physical products covering the
whole Baltic Sea area contain data with hourly resolution and 25 vertical
levels. The biogeochemical data are served with 6-h resolution and 25
vertical levels. For both products, the horizontal grid step is regular in
latitude and longitude and is approximately 1 nautical mile. The physi-
cal product is based on simulations with the HBM ocean model code
(HIROMB-BOOS-Model). The biogeochemical product is based on simu-
lations with the BALMFC-ERGOM version of the biogeochemical model
ERGOM, originally developed at IOW, Germany. The BALMFC-ERGOM
version has been further developed at Danish Meteorological Institute
(DMI) and Bundesamt für Seeschifffahrt und Hydrographie (BSH). The
BALMFC-ERGOM model is run online coupled with the HBM ocean
model code. In the analyses presented here, annual averages of salinity
and current velocity and summer averages (June to August) of temper-
ature and chlorophyll aconcentration were used.
In addition to the aforementioned data layers, depth data acquired
from the Baltic Sea Bathymetry Database (Baltic Sea Hydrographic
Commission, 2013) were used as a modelling input variable for
predicting blue mussel presence and growth. The locations of hard bot-
tom areas were obtained from the EMODnet portal (http://www. and unpublished sediment data were collated from Finn-
ish Environment Institute, Geological Survey of Sweden, and the
Bundesamt für Seeschifffahrt und Hydrographie. Wave exposure data
were produced by Aquabiota, using the Simplied Wave Model method
(SWM; Wijkmark and Isæus, 2010). The SWM method calculates the
wave exposure for mean wind conditions using a nested-grids tech-
nique to take into account long distance wind effects on the local
wave exposure regime. This method results in a pattern where the
fetch values are smoothed out to the sides, and around islands in a sim-
ilar way that refraction and diffraction make waves deect around
islands. Then a depth-attenuation correction was applied to the SWM
in order to estimate depth-attenuated wave exposure (Bekkby et al.,
2008). For maps of environmental variables, see Supplementary Fig. 1.
2.7. Modelling the occurrence of blue musselreefs along environmental gra-
dients of the Baltic Sea
In the case of distribution data, all samples having positive coverage
or biomass were considered as indicative of mussel presence and all
other samples were considered asabsences. The occurrence probability
of wild blue mussels on seaoor was modelled as a function of depth, sa-
linity, temperature, wave exposure and the presence of hard or mixed
substrate with sand, boulders and bedrock. These substrate types are
known to be good habitats for blue mussels in the Baltic Sea area (e.g.
Westerbom, 2006). A binomial Generalized Additive Model (GAM)
with logit link function was used for modelling occurrence. Possible
over-tting was limited by constraining the degrees of freedom of
model covariates.
2.8. Hydrodynamic connectivity model
The connectivity structure among all mussel reefs in the Baltic Sea
area was estimated with a biophysical model of larval dispersal. Blue
mussel larvae may drift in the water column for up to 30 days (Bayne,
1965). Thebiophysical model combinedow elds from an ocean circu-
lation model with a Lagrangian particle-tracking model simulating
transport of individual larvae from spawning to settling locations. The
ocean current velocity elds were produced with the three-
dimensional NEMO-Nordic model (Hordoir et al., 2013, 2015), a re-
gional conguration of the NEMO ocean engine (Madec, 2010)covering
the Baltic Sea, the Kattegat, the Skagerrak, and most of the North Sea.
The model has a horizontal spatial resolution of 3.7 km and 84 vertical
levels with depth intervals of 3 m at the surface and 23 m for the
deepest layers. The model has open boundaries between Cornwall and
Brittany, and between the Hebrides Islands and Norway with tidal har-
monics dening sea surface height (SSH) and velocities, and Levitus cli-
matology dening temperature and salinity (Levitus and Boyer, 1984).
The applied model had a free surface and the atmospheric forcing was
based on the re-analysis dataset ERA40 (Uppala et al., 2005). Runoff
was based on climatological data from several databases for the Baltic
Sea and the North Sea. Validation of the NEMO-Nordic showed that
the model correctly represents both tidally induced and wind driven
SSH anomalies (Hordoir et al., 2015).
To simulate larval drift trajectories, the Lagrangian particle-tracking
model TRACMASS (De Vries and Döös, 2001), that calculates transport
of particles using stored ow eld data from the ocean model, was
used. The velocity, temperature and salinity were updated with a regu-
lar interval for all grid boxes in themodel domain(in this study- every
three hours), and thetrajectory calculations wereperformed with a 15-
min time step. Particles simulating larvae of blue mussels were released
from the model grid cells (3.7 × 3.7 km
) that overlapped with the mus-
sel reef areas. From each grid cell, 294 particles were released on three
occasions between June to July as this time corresponds to a planktonic
larval phase of blue mussels in the Baltic Sea region (Kautsky, 1982).
Each larva was forced to drift in one of three depth intervals:25% of lar-
vae between 0 and 10 m, 50% of larvae between 10 and 15, and 25% of
larvae between 15 and 30 m (Corell et al., 2012). The pelagic larval du-
ration (PLD) was set to either 20 or 30 days with equal probability, and
settlement was assumed at the location when the PLD was completed.
All these simulations were repeated for 8 years (19952002),
representing a range of North Atlantic oscillation index values (NAO;
Hurrel and Deser, 2009), which is known to correlate well with the var-
iability in circulation pattern, making a total of 670,000 released parti-
cles. A grid cell was considered to receive recruits if larvae spawned at
any of the reefs in the Baltic Sea range settled at the specied grid cell
(Supplementary Fig. 2).
2.9. Modelling the growthof blue mussels along environmental gradients of
the Baltic Sea
Blue mussel growth was modelled as statistical relationships be-
tween environmental variables and mussel growth yield experimen-
tally evaluated all over the Baltic Sea region. Only the environmental
variables known to affect regional patterns of Baltic Sea mussel growth
(salinity, temperature, chlorophyll a, exposure to waves) were included
in the model. It was assumed that new larvae can settle from 1st to 30th
of June and only in the grid cells that are connected to mussel reefs (see
previoussubsection). Thegrowth simulations were based on dry weight
of mussels as opposed to length (this allowed for negative growth dur-
ing periods of resource limitation and for greater exibility when deal-
ing with gamete production). The model assumed that the new larvae
appeared in June. Yields were normalized with the total incubation
time (to produce data for yield per day) but a linear pattern was ob-
served within a year, thus allowing to extrapolate the predictions to
365 days. Two year predictions were calculated from one-year predic-
tions using a coefcient obtained from individual growth patterns.
Gaussian GAMs with an identity link function were used for model-
ling. Possible over-tting was reduced by constraining the degrees of
freedom of model covariates. Final growth model included salinity and
interaction between wave exposure and chlorophyll a.Tworandomfac-
tors were used to model the dependence inherent in the growth data.
First, for a combination of farm area and year to allow for yearly varia-
tion in different farming areas. Second, for a combination of place
6J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
(within area) and yearto allow for yearly variation within areas. Thus it
was assumed that yields for two farms from thesame area or place from
the same year are more similar than yields for two farms from different
areas or places from the same year. To normalize the residual distribu-
tion, the yield per day was fourth-root transformed.
For prediction, it was assumed (based on empirical knowledge) that
no further gain can be obtained from wave exposure values above
200,000 which represents a transition from moderately exposed to ex-
posed areas (Kotta et al., 2015). Blue mussel growth was deemed im-
possible at salinity values below 3.5 (Riisgård et al., 2014). The
available growth data was more evenly spread along the north-
eastern and south-western coasts as compared to central coasts of the
Baltic Sea. Hence, in these sparsely sampled areas, growth for locations
far from any growth assessment was estimated by spatial extrapolation.
However, with respect to salinity, the main factor explaining mussel
growth in our model, extrapolations are not extensive, since thegrowth
data spans the salinity gradient.
To quantify meaningful effect sizes of the two components (salinity
vs chlorophyll aand exposure to waves) in the study area, predictions of
two-year yield were obtained for six different combinations of predictor
values, as follows. First, for each predictor the 2.5% quantile (low), the
median and the 97.5% quantile (high) were determined. To assess the
interaction effect of chlorophyll aand wave exposure, salinity was
kept at its median value while four different value combinations (low-
low, low-high, high-low, high-high) were assigned to the other two
predictors. To assess the effect of salinity, the other two predictors
were keptat their respective medians while two different value combi-
nations (low and high) were assigned to salinity.
2.10. Nutrient removal at harvest
The mass of N and P removed during harvest at the three farms was
estimated by multiplying reported wet weight harvest values and least
squares mean estimates for other seasonwet weight nutrient per-
centages for the three regions of the Baltic. These estimated percentages
were obtained from analyses of variance(ANOVA) of N and P tissue con-
centrations from 124 composite samples (Supplementary Tables 2 and
3. Results
3.1. Analyses of farmed mussels
A total of 9478, 1516 and 4912 mussel samples were harvested and
measured from farms in the Outer, Central and Inner Baltic Sea (three
major analysis regions, Fig. 2). Average densities and individual wet
weights (±SE) of harvested mussels were 2654 ± 77 individuals m
(individuals per metre of rope) and 0.50 ± 0.02 g ww (wet weight) in
the Outer Baltic; 4998 ± 329 individuals m
and 0.20 ± 0.01 g ww
in the Central Baltic and 2326 ± 24 individuals m
and 0.16 ±
0.001 g ww in the Inner Baltic, respectively.
In total, 124 composite samples ofwhole mussels (shell andsoft tis-
sue) were availablefor dry matter and nutrient analysis. Samples of blue
mussels from the Outer Baltic had signicantly higher dry matter con-
tent (42.5%) than mussels from the Central (34.0%) or Inner (32.6%) Bal-
tic (Supplementary Table 1, Fig. 3). Region, season and their interaction
accounted for 62.3% and 67.7% of the total observed variation in mussel
tissue N and P percentages (Supplementary Tables 2 and 3). Nitrogen
concentrations were highest in Outer Baltic samples from spring
(1.23%), followed by those obtained for the same region in other sea-
sons (0.89%). There were no signicant differences among spring Cen-
tral Baltic samples (0.70%), Central Baltic samples from other seasons
Fig. 3. Mean and standard errors (bars) of the mean dry weight proportion in the Outer,
Central and Inner Baltic Sea. Letters depict statistically signicant differences between
groups (Tukey's HSD test results at pb0.05) and levels are statistically signicantly
different if they do not have any letters in common.
Fig. 4. Mean (lledshapes) and standard errors(bars) of nitrogen(N) and phosphorus(P) content in farmed musselsexpressed as a percentage ofwet weight in springvs other seasonsin
the Outer,Central and Inner BalticSea. Letters depictstatistically signicant differences betweengroups (Tukey'sHSD test results at p b0.05) with capital lettersdenoting N and lowercase
letters P groups. N and P levels are statistically signicantly different if they do not have any letters in common.
7J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
(0.69%), and spring Inner Baltic samples (0.57%). Inner Baltic samples
from other seasons (0.52%) were signicantly lower compared to all
other contexts except Inner Baltic spring samples (Supplementary
Table 2). Observed phosphorous concentrations followed similar trends
to those of N (Supplementary Table 3). The highest P concentrations
were measured in spring Outer Baltic samples (0.111%) and the lowest
in mussels from the Inner Baltic in other seasons (0.040%). Phosphorous
concentrations in spring Central (0.060%) and Inner Baltic (0.050%)
samples were signicantly lower than in spring Outer Baltic samples
and did not differ from Outer Baltic samples obtained in other seasons
(0.065%). As for N, P concentrations did not statistically differ between
seasons in mussels from the Central (spring: 0.060%, other: 0.053) and
Inner (spring: 0.050%, other: 0.040%) Baltic (Fig. 4).
Biomass yield and economic information were available for three
production farms: Kumlinge (Inner Baltic), Sankt Anna (Central Baltic)
and Kiel (Outer Baltic) (Table 1). Although the higher salinity and chlo-
rophyll alevels at Kiel may suggest a greater potential for mussel bio-
mass production and hence nutrient removal, this difference was not
manifested. In fact, nutrient removal was higher in Sankt Anna
(23.3 g N m
and 1.8 g P m
line) than either Kiel (22.2 g N m
and 1.6 g P m
line) or Kumlinge (3.7 g N m
and 0.3 g P m
Production costs were approximately four times higher at Kiel (1.85
kg biomass harvested
) than at Sankt Anna (0.52 kg biomass
). Costs were much higher at Kumlinge (8.52 kg biomass
). Differences in production costs were the main driver of
the large difference in nutrient removal costs which were lowest at
Sankt Anna (76 kg N)
and 981 kg P
) and higher at the other
two farms.
3.2. Modelling of biomass yield and regional nutrient removal
Spatially explicit estimates of farm biomass yield were predicted
as a function of site salinity, exposure to waves and food availability
(i.e. chlorophyll aconcentration) (Fig. 5). The model explained 82.3%
of the variation in the data. Modelled patterns of biomass yield were
driven by salinity at the regional scale and food availability at the
Fig. 5. Overview of the blue musselbiomass yield model and obtainedresponse curves.Estimations referto biomass yields obtained two yearsafter the establishment of the farms.Panel A
shows the interactive effects of exposure to waves and chlorophyll aconcentration (i.e. food availability) at a salinity of 7.5 psu. Panels B-D show the interactive effects of exposure to
waves and salinity at low, medium and high chlorophyll aconcentrations. Panels E-G show the interactive effects of salinity and chlorophyll aat low, medium and high exposure
levels. (For interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article.)
8J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
local scale. On the model scale, the effect of salinity was estimated to
be linear (and positive) which translated to a quartic effect on the re-
sponse scale. The interaction between food concentration and expo-
sure to waves (a good proxy of water movement and exchange) was
more complex. At low water movement, elevated chlorophyll acon-
centrations were associated with low biomass yield of mussels,
whereas at moderate to good water exchange, increasing chloro-
phyll aresulted in the raised biomass yield. The overall effect size
of salinity was about 13 times as large as the effect size of the afore-
mentioned interaction. The random effects, accounting for the inter-
annual and spatial variation not explained by the mean trends in sa-
linity and the interaction between wave exposure and food availabil-
ity, explained approximately 50% of the total variance. The model
does not simulate disastrous loss of harvestable mussel biomass as-
sociated with severe storms or harmful algal blooms.
Response curves predicting mussel yield as a function of environ-
mental conditions (Fig. 5) were combined with spatial data on salinity,
wave exposure and surface chlorophyll aconcentrations to produce
pan-Baltic estimates of potential rates of biomass removal that can be
obtained using farmed blue mussels (Supplementary Fig. 3). The
model extrapolation power was assessed by predicting the average
yield in the Kiel mussel farm that was not used for model tting. On
the model scale, the average yield was predicted to be only 10% smaller
than what was actually measured. Higher growth was predicted at
higher salinities and/or better food regimes, i.e. the Outer and Central
Baltic. Predicted biomass yield was highest in high-salinity areas of the
Fig. 6. Longline removed from the waterat Sankt Anna (A) showingmussel growth. Viewof mussel farm at Kiel showingoats on which linesare suspended (B).Panels (C) and (D) show
the environment underneath thesame farms. Blue surfaces of panels(E) and (F) show variability inoxygen conditions(mg l
) measuredat the sediment-water interface underneath the
farms and dottedlines indicatethe region of hypoxia and normoxia (for further data see (For interpretationof the references to colourin
this gure legend, the reader is referred to the web version of this article.)
9J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
Outer Baltic where it was estimated at 15 kg m
per harvest. With de-
clining salinity, predicted biomass yields varied between 1 and
in the Central Baltic. In marginal (Inner Baltic) regions, pre-
dicted biomass yields never exceeded 1 kg m
. Patterns in modelled
rates of biomass removal are similar to the observed patterns of N and
P content (Fig. 4), which are higher in the Outer Baltic than in either
the Central or Inner Baltic.
The harvest data from the three example farms(Table 1) can be put
into a regional context by comparing them to regional modelled rates of
biomass removal (Supplementary Fig. 3). The observed harvest at
Kumlinge is similar to model projections while the actual harvest at
Sankt Anna is 2.5 times larger than model projections and the Kiel har-
vest is 5 times smaller (Table 1).Mussels grow larger inwaters that are
more saline and therefore their substrate (i.e., the settlement rope) be-
comes quickly saturated and competition between mussels for space is
high. A multi-layered structure of large mussels is very unstable and
even moderate storms can remove outer layers from suspended ropes.
Many detached mussels were observed underneath the Kiel farm,
whereas nosuch losses were recorded in Sankt Anna (Fig. 6). Neverthe-
less, the full potential of high-salinity areas is demonstrated based on
harvest data from experimental fuzzy ropes deployed in Kiel farm
over a year and such substrates hosted nearly 10 kg mussels m
plementary Fig. 4).
When evaluating the potential of mussel cultivation as a mitigation
tool to reach regional nutrient reduction targets, a surprisingly small
marine area would need to be used for mussel farms in order to close
the remaining nutrient reduction gaps, i.e., 900 km
for the Central Bal-
tic and 600 km
for the Inner Baltic (Fig. 1). When farms are established
at optimal growth locations and optimal density, then nutrient removal
during mussel harvest can compensate up to 100% of the local and a
large part of the regional nutrient loading. Although the modelling re-
sults presented here suggest a higher efciency of mussel farms at
high salinity, the factual evidence suggests that Outer Baltic farms are
not necessarily more efcient in nutrient removal as compared to Cen-
tral Baltic farms. Importantly, the predicted total area of farms needed
for achieving HELCOM nutrient reduction targets could be achievable
under the current marine spatial planning regime in the Baltic Sea.
4. Discussion
Marine eutrophication is a pervasive and growing threat to global
sustainability (Conley et al., 2009b). While all reasonable efforts to re-
duce nutrient inputs from land to sea must continue, internal measures
are also needed to ensure the timely recovery of eutrophicated systems
(Savchuk, 2018). Extractive harvesting of farmed nativebivalve species,
including M. edulis/trossulus, is a sustainable, low-impact (Petersen
et al., 2014, 2019), circular (Spångberg et al., 2013) and potentially
cost-effective (Gren et al., 2009) internal measure for eutrophication
control (Suplicy, 2018). While arguments have been made against the
use of internal measures such as mussel farming (Stadmark and
Conley, 2011) or geo-engineering (Conley, 2012), there can be little
doubt that internal measures must be considered when all feasible ex-
ternal measures for nutrient load reduction have been explored, applied
and found to be inadequate or insufcient.
The BalticSea is a plausible representationof the likely future stateof
other coastal seas globally (Reusch et al., 2018) and the accumulated
knowledge for this region may serve as a useful future management
model for other internationally managed seas. Mitigation has already
been largely successful for recovery of Baltic Sea top predators
(Reusch et al., 2018) and some sh stocks (Eero et al., 2012). External
loads of both N and P from the surrounding catchment have declined
(Reusch et al., 2018;Savchuk, 2018) but average N concentrations are
decreasing slowly, if at all, while P concentrations continue to increase
(Savchuk, 2018). This mismatch between the successful reduction of
terrestrial nutrient inputs and failure to observe corresponding im-
provements in water column nutrient concentrations is due in part to
the ongoing release of nutrients accumulated in marine sediments
(Vahtera et al., 2007).
The predicted nutrient removal by mussel harvesting largely follows
the spatial patterns of mussel growth, i.e., farms in the Outer Baltic are
expected to have higher yields than in other Baltic Sea regions. Harvest
weight (kg m
) is linearly related to mussel size (Nielsen et al., 2016)
and blue mussels do not grow as rapidly in brackish waters as they doin
fully marine environments. While the small size of harvested mussels
poses challenges for feed or food production, the data presented here
suggest that the overall potential for nutrient removal does notdiminish
along the salinity gradient, except for the innermost parts of the Baltic
Sea (Table 1, Supplementary Fig. 3). While it is important to prioritize
high salinity sites in order to enhance the yield, even at reduced salin-
ities in the central Baltic Sea, a one hectare mussel farm with a density
of appropriate ropes may yield hundreds of tons of biomass per harvest
cycle. Furthermore, the harvest strategy can be optimised as smaller
mussels may be more efcient at nutrient removal due to lower detach-
ment rates asdensity dependentlosses can reach 50% in oceanic regions
with high biomass production (Haamer, 1996).
Unlike earlier studies (Dahlbäck and Gunnarsson, 1981;Hartstein
and Stevens, 2005), the monitoring of all existing mussel farms in the
Baltic Sea region offers no evidence to suggest that blue mussel farms
in the BalticSea have any negative effects on the local oxygen conditions
at the sedimentwater interface (Aigars et al., 2019). While others have
suggested that mussel farms can cause promote lower sediment oxygen
concentrations associated with a reduction in bioturbation or excessive
accumulation of organic matter(Stadmark and Conley, 2011), the oppo-
site phenomenon was observed at the Kiel mussel farm where an in-
crease in bioturbation led to higher sediment oxygen concentration
(Aigars et al., 2019). When sediments remain oxygenated, there is un-
likely to be any additional internal loading of P. However, oxygenated
conditions in the sediment under farms can suppress denitrication
(Carlsson et al., 2012).
Shellsh farming generally has lower environmental impacts than
other forms of aquaculture (Forrest et al., 2009;Kraufvelin and Díaz,
2015). Farmed blue mussels do not require any nutrient external inputs.
This means that unlike other forms of aquaculture, all of the nutrients
removed during harvest make a positive contribution to regional eutro-
phication reduction and a valuable regulative ecosystem service in eu-
trophic waters (Suplicy, 2018;Petersen et al., 2019). However, the
potential for localised nutrient enrichment in the immediate vicinity
of mussel farms does exist in very sheltered areas (e.g., Stadmark and
Conley, 2011;Holmer et al., 2015) and in such areas the possibility of
undesirable local eutrophication must be recognised and addressed.
Furthermore, farms can provide additional habitat for colonization
to supplement natural mussel reefs lost to anthropogenic impacts, espe-
cially human-facilitated invasion impacts of benthic predators. In the
Baltic Sea, the most relevant invasive predator is round goby, which
causes large-scale losses of benthic blue-mussel populations
(Skabeikis et al., 2019), e.g. one case-study location is estimated to
have lost 23% of its 230km
pre-invasion mussel reef area due to
round goby predation (Liversage et al., 2019). Suspended mussels will
attract negligible predation from such benthic predators, thus mussel
farming will help restore overall population levels. If a switch does
occur from natural mussel reefsto suspended farm mussels, this may in-
volve a reduced local-scale per-capita impact on eutrophication because
material excreted from suspended mussels will have greater dispersal
and dilution by water movements (Hartstein and Stevens, 2005) rather
than direct benthic retention. In addition, aquaculture activities often
produce benthic shell debris deposits (Sanchez-Jerez et al., 2019)
which increase sediment porosity and oxidised sediment layer depth,
as well as infaunal bioturbation (Zaiko et al., 2010). These benets
may be expected following extended establishment of mussel farms.
Using mussel farmingas an internal measure to mitigate eutrophica-
tion in the Baltic requires the development of appropriate legislative in-
struments (Ozoliņa, 2017) and resolution of sea-use conicts along
10 J. Kotta et al. / Science of the Total Environment 709 (2020) 136144
maritime spatial planning process (Kannen, 2014). The model-
predicted locationsof mussel farms for achieving eutrophication reduc-
tion targets do not take multiple sea-use conicts into account, espe-
cially tourism and sheries (Lindahl et al., 2005). While maritime
spatial planning tools for optimizing interests of various stakeholders
are well developed, thetools do not yet incorporate the implementation
of mussel farming. Careful planning of large-scale mussel farming could
avoid unacceptable environmental impacts or conicts with other uses.
Farms should be located in semi-exposed or exposed areas with good
water circulation where negative local effects to benthic habitat quality
are unlikely. Additionally, predation can compromise the production of
bivalves in otherwise suitable areas. Therefore, the risk of losing bio-
mass to, e.g., the eider ducks (Somateria mollissima) in the Outer Baltic
must be assessed before initiating a full-scale mussel production.
Other technical challenges including storms, epiphytes, and in some re-
gions ice, will also need to be considered and lessons learnt from previ-
ous mussel farming programmes need to be applied (National Research
Council, 2010). Furthermore, farm technology adapted to the culturing
of small mussels should be used whenever possible to maximize yields.
Blue mussel farming as a mitigation measure is particularly efcient to
counteract diffuse nutrient emissions as to date there are few other ef-
fective options to remove nutrients that have already reached the sea.
Commercial mussel farming can also contribute to rural sustainability
by providing jobs in economically depressed areas. It may also contrib-
ute to a clean-up of the local marine environment with benets for local
tourism, recreation and other cultural ecosystem services.
5. Conclusions
Eutrophication is a leading cause of impairment of many aquatic
ecosystems globally. While external measures to control nutrient inputs
must be pursued, there is also a need for internal measures in order to
restore water quality and enable ecosystem recovery in a timely man-
ner. Blue mussel farming is a promising low-impact and native
species-based internal method for eutrophication control in the Baltic
Sea and beyond. Mussels lter the water for phytoplankton and trap nu-
trients which are then removed from the aquatic environment through
harvest, allowing nutrient reuse as part of the circular economy. Blue
mussel farming in the Baltic Sea not only provides a tool for nutrient
mitigation, but also contributesto the social and economic sustainability
of rural areas. These results presented here provide factual data to sup-
port political decisions on internal measures for eutrophication control
and promote the sustainability of the Baltic Sea region through mussel
farming for nutrient management.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inu-
ence the work reported in this paper.
A vast majority of data was collected and the data analyses were
made under the INTERREG project Baltic Blue Growth. In addition, JK,
AK, KL, MR, KH and HOK were supported by the European Union Sev-
enth Framework Programme for research, technological development
and demonstration though BONUS MARES project. ED and PaK were
funded by the project Baltic EcoMussel (EU Central Baltic Interreg IVA
Programme 20122013) and PaK additionally by Svenska Kulturfonden
(Carl Cedercreutz stipendiefound) both in 2013 and in 2014. FRB ac-
knowledges the nancial support of the German Academic Exchange
Service (DAAD) - Doctoral Programmes in Germany 2015/16 (grant
57129429). PRJ was supported by The Searesearch program at the
Åbo Akademi University Endowment. EV acknowledges the SmartSea
project (Grant no. 292985). ML and PB were supported by the BONUS
project OPTIMUS - Optimization of Mussel mitigation culture for sh
feed in the Baltic Sea through the Swedish Agency for Marine and Water
Management contract no 4356-2016.
Author contributions
Conceived the study and wrote the paper: JK, MF,AK, KL, LB, PRJ. Col-
lected data: MR, FRB, PB, IB, ED, SK, PaK, PeK, OL, ML, MML, MM, ANS,
HOK, MO, SK, JR, AŠ, EV, HS. Obtained funding and analysed data: JK,
MF, AK, KH, PRJ, AV. All authors discussed the results and edited the
Data availability
The datasets that were generated and/or analysed during the current
study are freely available from the corresponding author on a request.
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... Here, low trophic aquaculture sectors such as shellfish and algae farming represent novel sustainable and environmentally restorative aquaculture trends [1]. Cultivated mussels are filter feeders and during harvest, a significant amount of excess nutrients can be removed from the marine environment [2,3]. Moreover, besides their nutrient sequestration potential, mussels act as nutrient sinks by ingesting particles suspended in the water column and thereby directly improving water quality. ...
... As in other ecosystems, the Baltic Sea is characterized by its legacy nutrients that result in adverse symptoms of eutrophication [5]. To date, mussel farming is considered one of the most promising measures to remove these excess nutrients from the Baltic Sea [3]. The most promising aquaculture species in the Baltic Sea is the blue mussel, a hybrid of the two species Mytilus edulis and Mytilus trossulus [6]. ...
... Earlier studies demonstrated that shellfish farming in the Baltic Sea is efficient, cost effective, and removes large amounts of nutrients [3,7]. Comprehensive environmental monitoring of all existing mussel farms in the Baltic Sea did not identify any significant negative environmental impacts in any aspect over a three-year period [7]. ...
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Mussel farming is a novel and growing aquaculture field in the Baltic Sea. Nevertheless, there is very little published evidence on the processing of shellfish biomass in the region. The aim of this study is to develop a methodology for the extraction of organic-rich fractions from small-sized blue mussels of the Baltic Sea region that is applicable and economically viable for the feed and food industry. The efficiency of mussel meat separation was evaluated using different processing, drying, and filtration techniques. The laboratory experiments have succeeded in finding a method that is operationally feasible and does not require overly complex and expensive laboratory settings. These trials also showed that the separation of meat from fresh or frozen mussels can be achieved by simple crushing and sedimentation methods and the extraction yielded a significant amount of mussel meat (7.6%) with a high protein content (3.2%, i.e., half of the total protein found in the used mussel-mass). It also appeared that the use of filtration is not practical because the protein loss was extremely high. In addition, filtration makes the process of dry-matter separation more complex, and costs are unlikely to be compensated by the energy saved in drying.
... Mussels are socioeconomically relevant for many coastal economies and play key functional roles in ecosystems. They can provide ecosystem services ranging from eutrophication control (Kotta et al., 2020), transferring energy from the pelagic to the benthic food chains, to adding complexity to ecological systems with their shell structures (Borthagaray and Carranza, 2007;Burlakova et al., 2012;Yurek et al., 2021). In Brazilian coastal environments, the brown mussel (Perna perna, Mytilidae) is an important member of the native biodiversity and the only farmed mytilid species in the country (Lopes and Fonseca, 2008), with many coastal populations already complementing their diet by extracting wild brown mussels from the environment. ...
... Impacted regions such as PR-1 can benefit from mussel farming as it provides other relevant ecosystem services (van der Schatte Olivier et al., 2018) such as bio-remediation and water clarification. In this case, farming can be used to control eutrophication and develop healthier environments (Kotta et al., 2020). Other than mussel production for food consumption, taking the ecosystem services provided by the proposed activity into account when designing and planning a site can ensure more efficient management in these regions, building a path towards a more sustainable and efficient land or water use (Gentry et al., 2020). ...
Aquaculture can play a key role in providing sustainable and low-cost protein sources, with the potential to help particularly the socially and economically vulnerable population. Although many coastal populations already complement their diet by extracting wild brown mussels (Perna perna, Mytilidae) from the environment, an explicit assessment for mussel growth potential along the Brazilian coast has been conspicuously lacking. We provide a large-scale assessment for prospecting and developing mussel culture by applying a Dynamic Energy Budget (DEB) model coupled with remote sensing. We estimated DEB parameters for the P. perna Brazilian population and used satellite-derived yearly data was used as forcing variables, containing information of chlorophyll-a concentration as a proxy for food concentration, sea-surface temperature to modulate metabolic performance, and particulate organic carbon - discounted the contribution of chlorophyll-a - to take into account the negative effect of particles in the mussel ingestion rates. We then simulated mussel growth along a large region of the Brazilian coast and obtained the time it takes for the mussel to reach a 5 cm market-relevant length within each pixel as a means to visualize mapped mussel growth potential indicating the time it takes to reach commercial length. Our results highlight the regions where mussel growth can be relevant for supporting subsistence livelihoods and also for securing income for local communities by performing economic activities, as many of the identified regions do not yet have active mussel culture sites. We also show that mussels can be used for ecosystem services in regions where farming for human consumption is not advisable. Our study provides further evidence that bioenergetic models coupled with remote sensing allow for a pragmatic and cost-effective path to assess growth performance along large regions with implications for developmental policy and spatial planning.
... LTM species can be grown with lower energy requirements and zero feed or fertilizer inputs, as they extract dissolved nutrients or planktonic/detrital foods directly from the marine environment, and yet are nutrient-dense food sources rich in protein, unsaturated fats and micronutrients (Wright et al., 2018;Hallström et al., 2019;Naylor et al., 2021). Furthermore, LTM aquaculture can also provide a range of valuable non-food ecosystem services such as biodiversity enhancement and eutrophication remediation (van der Schatte Olivier et al., 2018;Gentry et al., 2019;Kotta et al., 2020;Cabre et al., 2021;Naylor et al., 2021;Theuerkauf et al., 2021;The Nature Conservancy, 2022) and may also transform linear nutrient flows from land to the sea into circular systems (Folke and Kautsky, 1992;Petersen et al., 2019;Filippelli et al., 2020;Thomas et al., 2021). Farming of extractive LTM species is one of the most efficient, low-input, low-carbon food production systems, especially when compared to the farming of terrestrial livestock (Hilborn et al., 2018;Gephart et al., 2020). ...
... The effects are most pronounced in nearshore areas, although the severity vary between regions. Extractive LTM aquaculture has been proposed as a tool to remediate this challenge (Petersen et al., 2016;Kotta et al., 2020;Naylor et al., 2021;Theuerkauf et al., 2021). The latter is reflected in the global movement in aquaculture toward developing financial instruments to recognize the roles of (restoration) aquaculture to enhance ecosystem goods and services (The Nature Conservancy, 2021; Barrett et al., 2022). ...
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To limit compromising the integrity of the planet, a shift is needed towards food production with low environmental impacts and low carbon footprint. How to put such transformative change towards sustainable food production whilst ensuring food security into practice remains a challenge and will require transdisciplinary approaches. Combining expertise from natural- and social sciences as well as industry perspectives, an alternative vision for the future in the marine realm is proposed. This vision includes moving towards aquaculture mainly of low trophic marine (LTM) species. Such shift may enable a blue transformation that can support a sustainable blue economy. It includes a whole new perspective and proactive development of policy-making which considers, among others, the context-specific nature of allocation of marine space and societal acceptance of new developments, over and above the decarbonization of food production, vis á vis reducing regulatory barriers for the industry for LTM whilst acknowledging the complexities of upscaling and outscaling. This needs to be supported by transdisciplinary research co-produced with consumers and wider public, as a blue transformation towards accelerating LTM aquaculture opportunities in a net zero-carbon world can only occur by considering the demands of society.
... Harvesting of mussel biomass removes nutrients from the ecosystem and has been suggested as a tool to combat eutrophication and improve water quality (Kotta et al., 2020;Timmermann et al., 2019). However, ecosystem effects of mussel farming such as changes in sediment nutrient release, and denitrification should be included in the nutrient budgets, because it may cancel out the nutrient removal obtained from harvesting (Hylén et al., 2021;Stadmark & Conley, 2011). ...
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Compared to land‐based sources, mussel aquaculture provides food products with a high‐quality protein content and a low carbon footprint. At the same time, mussel cultures store nutrients in their tissue that are removed from the system through harvesting. However, increasingly development of suspended bivalve aquaculture in the coastal zone also comes with a price as the ecological carrying capacity of the ecosystem may be exceeded. The present study aims to support future fjord‐management by estimating the nutrient budgets and ecological impacts of intensified mussel aquaculture in a shallow semi‐enclosed system, the Limfjorden, using 3D ecosystem modeling. Model results showed a net removal of nutrients by suspended mussel cultures at basin scale, whereas at farm scale the efficiency was lower due to increased sediment fluxes. An increase in mussel farming from the current 4 kt‐fresh weight to a future projection of 104 kt‐fresh weight did not exceed the ecological carrying capacity with respect to impacts on sediment chemistry but could cause local declines in benthic bivalve populations. Intense mussel farming provided ecosystem services such as better oxygen conditions and higher Secchi depth together with lower nutrient‐ and chlorophyll a concentrations on basin scale. In addition, there was a redistribution of nutrients, chlorophyll a concentrations, and Secchi depth between sub‐basins in the fjord depending on farming location and intensity. Overall, intensified mussel farming could contribute to the mitigation of eutrophication effects by removing nutrients from land sources and by reducing the local sediment loading.
... It has been proposed that mussel farming, when optimized for mitigation purposes, has the potential to reduce nitrogen concentrations at a competitive cost when compared to land-based measures Gren et al., 2009Gren et al., , 2018Kotta et al., 2020;Petersen et al., 2016b;Petersen et al., 2014). Gren et al. (2009) examined the costeffectiveness of including mussel farming in combination with 20 other abatement and mitigation measures across 24 basins in the Baltic Sea and concluded that, for a wide range of reduction targets, mussel farming reduced total costs. ...
Implementation of effective policy instruments to reduce emissions from non-point agricultural sources has been challenging. Mussel farming has the potential to mitigate diffuse nitrogen losses from agricultural production, and a Water Quality Trading Market (WQTM) between agricultural and mussel farmers could potentially be an efficient mechanism. We simulate a hypothetical WQTM in a catchment in northern Denmark using a relatively new approach referred to as a smart market for water quality. Building on previous work, we integrate mussel farmers as nitrogen permit sellers in a WQTM involving agricultural farmers and analyze the effect of the market on the cost of meeting water quality improvement targets. In addition, we set-up scenarios with decreasing levels of participation by agricultural farmers (−10%, −20% and − 30%). The results show a clear benefit from allowing trading between agricultural and mussel farmers, reducing the total costs by as much as 11.9%. Lower participation results in reductions in the benefits from trade. However, allowing mussel-based mitigation to supply N permits at modest cost can potentially partially circumvent the well-documented challenge that agricultural farmers are reluctant to act as N permit suppliers in WQTMs. The study illustrates the economic and environmental potential of integrating land- and marine-based farmers in a joint policy scheme to reduce nitrogen emissions to coastal and marine areas.
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Marine aquaculture has the potential to increase its contribution to the global food system and provide valuable ecosystem services, but appropriate planning, licensing and regulation systems must be in place to enable sustainable development. At present, approaches vary considerably throughout the world, and several national and regional investigations have highlighted the need for reforms if marine aquaculture is to fulfil its potential. This article aims to map and evaluate the challenges of planning and licensing for growth of sustainable marine aquaculture. Despite the range of species, production systems and circumstances, this study found a number of common themes in the literature; complicated and fragmented approaches to planning and licensing, property rights and the licence to operate, competition for space and marine spatial planning, emerging species and diversifying marine aquaculture production (seaweed production, Integrated Multi-Trophic Aquaculture [IMTA], nutrient and carbon offsetting with aquaculture, offshore aquaculture and co-location and multiuse platforms), and the need to address knowledge gaps and use of decision-support tools. Planning and licensing can be highly complicated, so the UK is used as a case study to show more detailed examples that highlight the range of challenges and uncertainty that industry, regulators and policymakers face across interacting jurisdictions. There are many complexities, but this study shows that many countries have undergone, or are undergoing, similar challenges, suggesting that lessons can be learned by sharing knowledge and experiences, even across different species and production systems, rather than having a more insular focus.
Phosphorus (P), one of the essential and indispensable elements for the growth of living organisms, is finite and nonrenewable for a multitude of applications. Economic P recycling can contribute to sustainable P management and lessen the pressure on P imports of the EU. Several European countries now have paved the way for establishing P recovery legislation and successively implemented compulsory recycling from P-rich wastes. This paper provided a comprehensive review for analyzing the potential of existing P recycling resources and engineering technologies in terms of their feasibility and applicability in Europe. Potential P-rich flows from the waste streams and eutrophic environments were deeply investigated. Regarding their technical principles, application, and availability, different engineering approaches for P release enhancement and P recovery were also reviewed in a critical manner. Moreover, assessments on engineering applications for P recycling considering economic feasibility and environmental benefits were also presented. Wastewater treatment plants show promising recovery efficiency, whereas industrial processes with sludge ashes as input allow up to 95% recovery of P. Collectively, in Europe, efficient and sustainable management of P has been recognized as a necessary strategy to meet the growing demand for mineral P.
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Aquaculture has been one of the fastest‐growing food production systems sectors for over three decades. With its growth, the demand for alternative, cheaper and high‐quality feed ingredients is also increasing. Innovation investments on providing new functional feed alternatives have yielded several viable alternative raw materials. Considering all the current feed ingredients, their circular adaption in the aquafeed manufacturing industry is clearly of the utmost importance to achieve sustainable aquaculture in the near future. The use of terrestrial plant materials and animal by‐products predominantly used in aquafeed ingredients puts a heavily reliance on terrestrial agroecosystems, which also has its own sustainability concerns. Therefore, the aquafeed industry needs to progress with functional and sustainable alternative raw materials for feed that must be more resilient and consistent, considering a circular perspective. In this review, we assess the current trends in using various marine organisms, ranging from microorganisms (including fungi, thraustochytrids, microalgae and bacteria) to macroalgae and macroinvertebrates as viable biological feed resources. This review focuses on the trend of circular use of resources and the development of new value chains. In this, we present a perspective of promoting novel circular economy value chains that promote the re‐use of biological resources as valuable feed ingredients. Thus, we highlight some potentially important marine‐derived resources that deserve further investigations for improving or addressing circular aquaculture.
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Multi-decadal monitoring data and archived biological samples make the rapidly changing Baltic Sea an ideal system for quantifying alterations in ecosystem function. The Baltic Sea blue mussel (Mytilus edulis trossulus) acts as a keystone species by providing food and habitat for other species, and through its filtering activity clearing the water and promoting organic matter and nutrient cycling between benthic and pelagic ecosystems. A decrease in biomass of blue mussels along with altered environmental conditions has been observed in the northern Baltic Sea over a 24-year period (1993–2016), but the size distribution of the mussels was unknown. The present study focuses on retrospective measurements of the individual shell lengths of these archived blue mussels. The mean shell length was found to be significantly lower at the end of the time series than in the beginning (11.1–11.5 mm for the years 1993–1996 compared to 10.3–10.9 mm during 2012–2015), with the proportion of larger mussels decreasing in the population. This 6% decrease in shell length translated into a significantly lower (16%) mean filtration capacity at the end of the time series compared to the beginning, influencing the ecosystem services blue mussels provide.
Nature-Based Solutions (NbS), a concept introduced in the late 2000s, has developed rapidly during the last years and is now frequently appearing in a broad spectrum of policies developed within the European Union. Its role in marine policies and research programmes is however still limited, but is likely to increase as NbS are adopted as key terminology in both biodiversity strategies and the EU taxonomy for sustainable financing. This will enhance the need for scientific advisory institutions to provide evidence-based advice on potential impacts of various combinations of marine NbS. To facilitate a critical debate about the prospects and pitfalls related to the operationalisation of marine NbS in an EU context, this paper provides an analysis of core definitions, potential categories of marine NBS and a suite of case studies. Coastal waters, shelf and open oceans present multiple options for testing new and scaling up known NbS, which could support both environmental restoration simultaneously with addressing multiple societal challenges, paving the way for a new level of ecosystem-based management. However, as the acceptance of NbS types will depend on ecosystem state and thus history, it will be a significant task to consistently communicate why some solutions may count as a NbS in some areas, while not in others. To conclude, the paper therefore raises a set of research priorities and policy advice aimed at ensuring the successful advice and deployment of marine NBS in support of multiple societal goals.
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Fish farm facilities become colonized by biofouling, and in situ cleaning activities may increase the accumulation of biofouling, mostly shell-hash, on the sediment. However, there is a lack of knowledge about the effect of fish farming on this process. We evaluated the effect of fish farming on shell-hash accumulation on sediments in three fish farms in the Western Mediterranean in Spain. On the one hand, coverage of non-degraded shell on the seabed was estimated using an underwater camera attached to a frame of 1 × 1 m. On the other hand, superficial sediment samples were taken by a Van-Veen grab, and from a subsample, shell-hash was sorted at the laboratory, dried, and weighted. A significant increase of shells on sediment was detected under fish farms compared with the other treatments, with average values of 53 g kg-1, and 1.12% of cover. Shell-hash at zones close to the fish farm cages (Zone of Influence located between 40 to 60 m from the closest cage) did not show statistical differences compared to the reference zones, 300–500 m away from the concession limits, but the shell cover showed statistical differences. Fish farming activities produce a local increase in the sedimentation rate of shells under the cages. The derived ecological consequences of this accumulation need to be further studied.
Technical Report
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2 LIMITATIONS OF USING BLUE MUSSEL FARMS AS A NUTRIENT REDUCTION MEASURE IN THE BALTIC SEA Blue mussel farming has been suggested as a cost effective nutri- ent mitigation tool in the Baltic Sea, to complement land-based measures. However, the environmental conditions of the Baltic proper, particularly the low salinity, affect the nutrient uptake efficiency of the blue mussels. Farming of mussels with the pri- mary aim to capture nutrients from the sea has also been ques- tioned due to the risk for unwanted environmental effects. In this report, we discuss potential problems regarding blue mussel farming as a nutrient mitigation measure in the Baltic proper, based on (1) the effects of low salinity on mussel growth and physiology and (2) potential negative environmental effects. The low salinity in the Baltic proper means that the mussels are under high physiological stress and results in a slow growth rate. In addition, blue mussels in the Baltic proper contain less meat and have a lower nutrient content compared to mussels living in higher salinity. Together, this leads to a lower nutrient uptake efficiency in Baltic proper blue mussel farms. In addition, the longer time to harvest in the Baltic increases risks for practical farming problems due to ice, storms, epiphytes and eider preda- tion affecting the small and weakly attached Baltic mussels. These factors have generally been underestimated, but should probably rather be regarded as normal for the Baltic proper. Holistic scientific studies of mussel farming nutrient uptake capacity have been conducted in areas with higher salinity in the Baltic Sea (>14 psu). But there are few data on harvest yield and nutrient content in blue mussel farms from the Baltic proper, which makes it difficult to provide a reliable estimate of the nut- rient abatement potential of mussel farms. Using available data, we estimate that nitrogen uptake efficiency per farming area and time may be 10 times lower in the Baltic proper than in blue mussel farms from areas with higher salinity. The phosphorus uptake efficiency is approximately five times lower than in hig- her salinity. This estimation needs to be refined when more data is available. Although it may be possible to reach higher uptake efficiency, it is clear that previously published estimates of miti- gation capacity of Baltic Sea blue mussel farms are too high. Mussel farming is regarded as relatively low-impact aquaculture, but blue mussel farms can have negative effect on the local environment, in particular if they are large or dense. For instance, mussel farms lead to local accumulation of nutrients and organic matter, which may result in oxygen deficiency on the seabed and unwanted plankton blooms. There is a need for more field studies and ecosystem modelling before the effects of large-scale farming for nutrient abatement in the Baltic proper can be properly evaluated. In conclusion, a realistic evaluation of the potential to use blue mussel farming as a nutrient abatement measure in the Baltic Sea needs to account for the low salinity and for the risk for environmental effects of large-scale farming. There is a need for more research supporting that blue mussel farming can provide a cost-efficient nutrient uptake in the Baltic proper, as well as research on the environmental effects of mussel farming, before it can be recommended as a measure to reduce eutrophication.
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Ecosystem services provided by marine bivalves in relation to nutrient extraction from the coastal environment have gained increased attention to mitigate adverse effects of excess nutrient loading from human activities, such as agriculture and sewage discharge. These activities damage coastal ecosystems and require action from local, regional, and national environmental management. Marine bivalves filter particles like phytoplankton, thereby transforming particulate organic matter into bivalve tissue or larger faecal pellets that are transferred to the benthos. Nutrient extraction from the coastal environment takes place through two different pathways: (i) harvest/removal of the bivalves – thereby returning nutrients back to land; or (ii) through increased denitrification in proximity to dense bivalve aggregations, leading to loss of nitrogen to the atmosphere. Active use of marine bivalves for nutrient extraction may include a number of secondary effects on the ecosystem, such as filtration of particulate material. This leads to partial transformation of particulate-bound nutrients into dissolved nutrients via bivalve excretion or enhanced mineralization of faecal material. In this chapter, concepts in relation to nutrient extraction by bivalves are presented and discussed in relation to nutrient cycling and additional effects of enhancing bivalve communities. In addition, methods to valorise nutrient extraction by bivalves are evaluated. Examples of calculations of the value of nutrient extraction by bivalves are presented.
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The invasive round goby has established a viable population within 9 years of its first introduction to Lithuanian coastal waters (SE Baltic Sea). During its expansion phase, abundances increased 23-fold, which led to the near complete eradication of its main prey, the blue mussel, at < 20 m depth. The round goby population showed a stabilizing trend after blue mussel biomass was depleted; however, their abundance has not declined. The round goby feeds efficiently on newly settled mollusks, causing a severe constraint for blue mussel recovery. Changes in blue mussel availability and size structure induced a dietary shift in wintering long-tailed duck towards fish prey. An energetically dense food source sustains a good body condition in long-tailed ducks, however the change in trophic position (from 3.1 to 4.3 trophic level) suggests the potential for a reduction in their carrying capacity. Results from this study also show that coastal habitats with low and unpredictable population dynamics of blue mussel become less attractive wintering sites for long-tailed duck in the Baltic Sea. We also document a cascading effect of invasive species in the food web.
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The oversupply of nutrients (phosphorous and nitrogen) in fresh and marine water bodies presents a serious ecosystem threat due to impacts on water quality through eutrophication. With agriculture characterized as a primary driver of eutrophication, the role of food consumption and trade has been the focus of recent phosphorus and nitrogen impact studies. However, the environmental impacts associated with non-food commodities are significant and yet to be characterized. Here, we link a spatially explicit treatment of phosphorous and nitrogen eutrophication potentials to a multi-regional input–output approach to characterize the importance of overall consumption for marine and freshwater eutrophication across 44 countries and 5 rest-of-world regions over the period 2000–2011. We find that clothing, goods for shelter, services and other manufactured products account for 35% of global marine eutrophication and 38% of the global freshwater eutrophication footprints in 2011, up from 31 and 33%, respectively, in 2000. Relative to food consumption, non-food consumption is also significantly more income elastic and shaped by trade. As economies develop, this points to the need for trade agreements and policies to consider the displacement of ecosystem impacts. As agriculture is the primary driver of eutrophication resulting from the oversupply of nitrogen and phosphorus to water bodies, much attention has been paid to the environmental impacts of food consumption. Little is known about the impacts of consuming other goods. This study shows that in 2011 the final demand for non-food products accounted for over one-third of the global marine and freshwater eutrophication impacts—a 28% increase since 2000.
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Coastal global oceans are expected to undergo drastic changes driven by climate change and increasing anthropogenic pressures in coming decades. Predicting specific future conditions and assessing the best management strategies to maintain ecosystem integrity and sustainable resource use are difficult, because of multiple interacting pressures, uncertain projections, and a lack of test cases for management. We argue that the Baltic Sea can serve as a time machine to study consequences and mitigation of future coastal perturbations, due to its unique combination of an early history of multistressor disturbance and ecosystem deterioration and early implementation of cross-border environmental management to address these problems. The Baltic Sea also stands out in providing a strong scientific foundation and accessibility to long-term data series that provide a unique opportunity to assess the efficacy of management actions to address the breakdown of ecosystem functions. Trend reversals such as the return of top predators, recovering fish stocks, and reduced input of nutrient and harmful substances could be achieved only by implementing an international, cooperative governance structure transcending its complex multistate policy setting, with integrated management of watershed and sea. The Baltic Sea also demonstrates how rapidly progressing global pressures, particularly warming of Baltic waters and the surrounding catchment area, can offset the efficacy of current management approaches. This situation calls for management that is (i) conservative to provide a buffer against regionally unmanageable global perturbations, (ii) adaptive to react to new management challenges, and, ultimately, (iii) multisectorial and integrative to address conflicts associated with economic trade-offs.
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The Baltic Sea is one of the world's marine areas well-covered by both long-term observations and oceanographic studies. It is also a large coastal area in which eutrophication had already been recognized half a century ago. While the mechanisms of eutrophication are largely understood, several features are less recognized and sometimes neglected, including: (a) natural and anthropogenic North-South and East-West nutrient gradients within the drainage basin and marine ecosystems; (b) the compensatory potential of the interconnectivity between the Baltic Sea basins; (c) long nutrient residence times and high buffer capacity of the system, resulting in slow responses to nutrient load reductions. Particularly important is the interaction of (d) naturally occurring saltwater inflows sporadically ventilating deep water layers and (e) a partly man-made intensification of biological oxygen consumption. Resulting redox alterations of biogeochemical nitrogen and phosphorus cycles are locked in a “vicious circle” that promotes cyanobacterial nitrogen fixation, thereby hindering nitrogen load reduction and sustaining an elevated trophic state. This tight coupling of natural environmental variation and human impacts complicates both scientific studies and management recommendations. Our primary objective is to describe all these features and mechanisms with the best available data on nutrient loads, and unique estimates of the basin-wide nutrient pools. These data are presented as both long-term time series and empirical nutrient budgets. The analysis is supplemented by results of biogeochemical modeling. A second, more practical objective is to make these time series available to the community.
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Beneath the waves, oxygen disappears As plastic waste pollutes the oceans and fish stocks decline, unseen below the surface another problem grows: deoxygenation. Breitburg et al. review the evidence for the downward trajectory of oxygen levels in increasing areas of the open ocean and coastal waters. Rising nutrient loads coupled with climate change—each resulting from human activities—are changing ocean biogeochemistry and increasing oxygen consumption. This results in destabilization of sediments and fundamental shifts in the availability of key nutrients. In the short term, some compensatory effects may result in improvements in local fisheries, such as in cases where stocks are squeezed between the surface and elevated oxygen minimum zones. In the longer term, these conditions are unsustainable and may result in ecosystem collapses, which ultimately will cause societal and economic harm. Science , this issue p. eaam7240
Incorporating ecosystem changes from non-indigenous species (NIS) is an important task of maritime spatial planning. Maritime spatial planning requires a framework that emphasises ecological functioning in a state of dynamic change, including changes to ecosystem services from functions introduced by new NIS. Adaptable modelling toolsets should be developed that can readily incorporate knowledge of new NIS. In the Baltic Sea, recent NIS examples are the North American mud crab Rhithropanopeus harrisii and the Ponto-Caspian round goby Neogobius melanostomus. We performed environmental niche modelling that predicted N. melanostomus will spread across large areas of the Baltic Sea coast while R. harrisii will be limited to regions with high temperature and low salinity conditions. We then performed a meta-analysis on literature showing effects in the Baltic Sea from these NIS and calculated the standardised effect-sizes on relevant ecosystem services. Half the impacts identified for N. melanostomus were considered to increase ecosystem service outcomes, while all R. harrisii impacts caused apparent decreases. Effect coefficients were incorporated into an online impact assessment tool developed by the Estonian Marine Institute. Users with or without science training can use the portal to estimate areas impacted and changes to natural assets (km²) caused by these NIS and cumulative effects from other pressure-types. Impact estimates are based on best available knowledge from manipulative and correlative experiments and thus form a link between science and management. Dynamic modelling techniques informed from varied ecological and methodological perspectives will effectively advise spatial planners about rapid maritime changes and mitigation actions to reduce NIS impacts especially in the focus areas.