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ORIGINAL RESEARCH ARTICLE
Microphytobenthic primary production on exposed
coastal sandy sediments of the Southern Baltic Sea
using ex-situ sediment cores and oxygen optodes
Kana Kuriyama
a
, Sigrid Gründling-Pfaffa
, Nora Diehl
b
, Jana Woelfel
a
,
Q1
Ulf Karsten
a , ∗
a
Institute of Biological Sciences, Applied Ecology and Phycology, University of Rostock, Germany
b
Institute of Biology and Chemistry, Marine Botany, University of Bremen, Germany
Received 26 August 2020; accepted 5 February 2021
Available online xxx
KEYWORDS
Benthic diatoms;
C/N ratio;
Respiration;
Hydrodynamics
Abstract The shallow coastal water zone of the tide-less southern Baltic Sea is dominated
by exposed sandy sediments which are typically inhabited by microphytobenthic communities,
but their primary production is poorly studied, and hence four stations between 3.0 and 6.2
m depth were investigated. Sediment cores were carefully taken to keep the natural layering
and exposed in a controlled self-constructed incubator. Respiratory oxygen consumption and
photosynthetic oxygen production were recorded applying planar oxygen optode sensors. We
hypothesized that with increasing water depths the effects of wind- and wave-induced erosion
and mixing of the upper sediment layer are dampened and expected higher microphytobenthic
biomass and primary production in the incubated cores.
Our data partly confirm this hypothesis, as cores sampled at the most exposed stations con-
tained only 50% chlorophyll a m
−2
compared to the deeper stations. However, primary produc-
tion was highly variable, probably due to fluctuating sediment-disturbing conditions before the
cores were taken. Due to these physical forces sand grains were highly mobile and rounded,
and small epipsamic benthic diatoms dominated, which preferentially occurred in some cracks
∗Corresponding author at: University of Rostock, Institute of Biological Sciences, Applied Ecology and Phycology, Albert-Einstein-Straße 3,
18057 Rostock, Germany.
E-mail address: ulf.karsten@uni-rostock.de (U. Karsten).
Peer review under the responsibility of the Institute of Oceanology of the Polish Academy of Sciences.
https://doi.org/10.1016/j.oceano.2021.02.002
0078-3234/ ©2021 Institute of Oceanology of the Polish Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access
article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
Please cite this article as: K. Kuriyama, S. Gründling-Pfaff, N. Diehl et al., Microphytobenthic primary production on exposed coastal
sandy sediments of the Southern Baltic Sea using ex-situ sediment cores and oxygen optodes, Oceanologia, https://doi.org/10.1016/j.
oceano.2021.02.002
K. Kuriyama, S. Gründling-Pfaff, N. Diehl et al.
ARTICLE IN PRESS
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and crevices as visualized by scanning electron microscopy. The data fill an important gap in
reliable production data for sandy sediments of the southern Baltic Sea, and point to the eco-
logical importance and relevant contribution of microphytobenthic communities to the total
primary production of this marine ecosystem. Oxygen planar optode sensor spots proved to be
a reliable, sensitive and fast detection system for ex-situ oxygen exchange measurements in
the overlying water of intact sediment cores.
©2021 Institute of Oceanology of the Polish Academy of Sciences. Production and host-
ing by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
1. Introduction 1
The Baltic Sea is a brackish young marginal sea in north- 2
ern Europe. The German coastline of the Baltic Sea is 3
roughly 2,000 km long ( Jurasinski et al., 2018 and refer- 4
ences therein), and the shoreline is characterized by a range 5
of highly diversified coastal forms from rocky cliffs to sandy 6
beaches. The southern Baltic Sea shoreline in particular ex- 7
hibits a highly dynamic geomorphology. Rock particles are 8
steadily eroding from cliff lines, transported by coastal par- 9
allel currents and deposited at other places, i.e. the pro- 10
cesses of abrasion and sedimentation lead to a loss of upland 11
relief and at the same time to the formation of spits, lagoon 12
systems, shallow subtidal flats and wind flats ( Karsten et al., 13
2012 ; Schwarzer, 1996 ). Besides these specific geomorpho- 14
logical processes, the Baltic Sea represents a non-tidal sys- 15
tem. The tidal range is usually only 12—15 cm in the western 16
Baltic Sea, and can be even lower in the eastern part (e.g. 17
Polish coast 5 cm), but wind direction and wind speed in 18
combination with atmospheric pressure might temporarily 19
induce high waves and change sea water levels ( Lass and 20
Magaard, 1996 ). Consequently, meteorological and hydro- 21
dynamic conditions shape the southern Baltic Sea coastline 22
resulting in many exposed shallow water sandy sites. 23
Coastal sediments are typically inhabited by microphyto- 24
benthic communities, which can make up about 30% of the 25
global total coastal primary production ( Ask et al., 2016 ; 26
Cahoon, 1999 ; Gerbersdorf et al. 2005 ; Schreiber and Pen- 27
nock, 1995 ). Benthic microalgal communities typically ex- 28
hibit a high diversity of taxa consisting of representatives 29
of euglenids, chlorophytes, cyanobacteria, dinoflagellates 30
and diatoms ( Colijn and De Jonge, 1984 ; Launeau et al., 31
2018 ; Sundbäck and Miles 2002 ). But at most sites the domi- 32
nant group are benthic diatoms ( Cahoon, 1999 ), which fulfill 33
important ecological functions in shallow marine inter- and 34
subtidal environments as they live at the sediment-water 35
interface and thus directly influence various exchange pro- 36
cesses between these compartments (e.g. Sundbäck et al., 37
2000 ). As primary producers they are responsible for a huge 38
proportion of carbon fixation ( Ask et al., 2016 ; Blasutto et 39
al., 2005 ; Cahoon, 1999 ; MacIntyre et al., 1996 ) and thus are 40
an important supplier of organic carbon to grazers as well as 41
sediment feeding macro- and meiofauna ( Middleburg et al., 42
2000 ; Oakes et al., 2010 ). The release of dissolved organic 43
carbon by the excretion of extrapolymeric substances (EPS) 44
is common for benthic diatoms and thus an important car- 45
bon supply for bacteria ( Aslam et al., 2012 ; Hanlon et al., 46
2006 ). In addition, since EPS are sticky these compounds 47
stabilize and modify sediment surfaces ( De Brouwer et al., 48
2005 ), thereby reducing hydrodynamic erosion and control- 49
ling vertical fluxes of oxygen and other elements at the 50
sediment-water interface ( Risgaard-Petersen et al., 1994 ). 51
Furthermore, benthic diatoms strongly influence bacterial 52
remineralization in the upper sediment layer as oxygen fuels 53
nitrification but inhibits denitrification ( Cook et al., 2007 ). 54
Released nutrients, in turn, are beneficial for benthic algal 55
growth. 56
Habitat conditions for microphytobenthic communities 57
can be highly diverse, and hence their biomass and produc- 58
tivity varies along multifactorial spatio-temporal environ- 59
mental gradients such as, for example, small-scale dynam- 60
ics of sediment grain-size distribution ( Orvain et al., 2012 ). 61
But also other physical and biological gradients such as 62
tides, bathymetry, topography, light availability due to tur- 63
bidity, deposit-feeders, or sediment nutrient stocks might 64
affect benthic diatom photosynthetic activity ( Haro et al., 65
2020 ; Jesus et al., 2009 ; Kromkamp et al., 1995 ). Benthic 66
diatoms live either epipsamic (attached to sediment grains) 67
or epipelic (in the interspaces between sediment grains) 68
in the uppermost millimeter of sediments ( Woelfel et al., 69
2007 ). The diatom lifestyle depends on the exposition (e.g. 70
wave energy and currents) and sediment type (e.g. grain 71
size) since exposed sandy sediments typically harbor rather 72
small-sized epipsamic species ( Vilbaste et al., 2000 ; Woelfel 73
et al., 2007 ) whereas the occurrence of larger epipelic taxa 74
is limited by sand-scouring processes ( Sabbe, 1993 ). Besides 75
seasonality and light conditions, mechanical stress acting 76
on the seafloor such as wind induced currents or waves are 77
important physical factors controlling the establishment of 78
such phototrophic biofilms. Frequent resuspension and de- 79
position of sediment particles at exposed sites lead to recur- 80
rent disturbance temporarily enhancing shading effects or 81
even the burial of diatom cells with negative consequences 82
for the development of microphytobenthic biofilms on top 83
of such sediments. On the other hand, raphid benthic di- 84
atoms are able to escape unfavorable conditions via vertical 85
migration in or out of the sediment ( Harper, 1969 ). Thus, it 86
is likely that these phototrophs are capable to recover from 87
disturbance events of sediments such as after storms, and 88
resume their photosynthetic activity immediately ( Wulff et 89
al., 1997 ). 90
Microphytobenthic communities were studied all over 91
the world ( Cahoon, 1999 ). The latter author compiled > 92
80 studies in his comprehensive review and concluded that 93
previous estimates on microphytobenthic primary produc- 94
tion had markedly underestimated their relevance and 95
contribution for coastal shallow water production. Overall, 96
Cahoon (1999) provided for the first time an annual global 97
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estimate of 5 ×10
8 tons C by microphytobenthic primary 98
production, and indicated marine areas which were rela- 99
tively well studied (i.e. temperate regions), while others 100
were grossly under-sampled, such as the polar regions at 101
that time. Based on 13 studies Glud et al. (2009) reviewed 102
the importance of microphytobenthic communities for the 103
Arctic shallow water zone and calculated an annual pro- 104
duction between 1.1 and 1.6 ×10
7
tons C. In the temperate 105
zone of Europe there have been many studies undertaken 106
on microphytobenthic primary production in tide-influenced 107
habitats, particularly in the mouth of estuaries and the 108
Wadden Sea (e.g. Colijn and Dijkema, 1981 ; Daggers et 109
al., 2018 ; Frankenbach et al., 2020 ; Joint, 1978 ; MacIntyre 110
et al., 1996 ; Virta et al. 2019 ), which all confirm their 111
ecological importance for soft bottom coasts. In contrast, 112
the Baltic Sea is much less studied, and hence only very few 113
data exist (Lagoons of German Baltic Sea coast: Meyercordt 114
and Meyer-Reil, 1999 ; Wasmund, 1986 ; Yap, 1991 ; Bay of 115
Gdansk, Poland: Urban-Malinga and Wiktor, 2003 ; Bay of 116
Riga, Estonia and Latvia: Vilbaste et al., 2000 ), which indi- 117
cate gross primary production rates between 0.2 and 41.8 118
mg C m
−2 h
−1 depending on the sediment (mud vs. sand), 119
water depth and season, and which is generally lower then 120
in tidal systems such as the Ems Dollard Estuary (10—115 121
mg C m
−2 h
−1
, Colijn and de Jonge, 1984 ). In addition, the 122
rather few Baltic Sea studies are rather old and had been 123
carried out under different environmental settings (sandy 124
beach vs. sheltered lagoon) using different methodological 125
approaches (
14
C vs . O
2
). Consequently, direct comparison 126
of the limited data might be difficult. 127
Therefore, the focus in the present study was on micro- 128
phytobenthic primary production at four stations at an ex- 129
posed sandy coastal site of the Southern Baltic Sea, north 130
east of Rostock in close vicinity to the peatland Hütelmoor 131
at water depths between 3.0 and 6.2 m. This site is char- 132
acterized by strong wind- and wave-induced mixing of the 133
upper sediment layer along the coastline ( Jurasinski et al., 134
2018 ). We hypothesized that with increasing water depths 135
the effects of these physically disturbing factors are damp- 136
ened and hence favour higher microphytobenthic biomass 137
and primary production. Intact sediment cores were taken 138
by SCUBA divers between April and July 2017, exposed in the 139
laboratory under controlled conditions and measured using 140
oxygen optodes. 141
2. Material and methods 142
2.1. Site description 143
The Hütelmoor sampling stations S21, S25, S41 and S45 are 144
located on near shore exposed sandy sediments at a north- 145
westerly oriented coastline ( Figure 1 ). They are strongly 146
influenced by westerly winds and the resulting near shore 147
east-west current. Water depth at the four sampling sta- 148
tions ranged from 3.0 to 6.2 m. Water surface temperature 149
measured in 2017 ranged from 8.9 °C in April to 17.6 °C in 150
June, and absolute salinity varied between 7.2 and 11.5 S
A
151
(S
A
: absolute salinity) ( Tab le 1 ). 152
There is no direct river/stream run-off or wastewater 153
discharge in front of the nature reserve Hütelmoor. The 154
nearest estuary is the river Warnow, 10 km west of the sam- 155
Table 1 Environmental data for the sampling stations in front of the Hütelmoor (Southern Baltic Sea coast, Germany). Samples were taken between April and July 2017.
Water depth (m), water temperature ( °C) and salinity (S
A
) were measured in the field at the respective stations. For water content (WC, % of fresh weight (FW)), organic
matter (OM, mg g
−1
dry weight (DW)) and carbon:nitrogen ratio (C/N, mol/mol) sediment cores were taken at different time points and processed in the laboratory. For these
parameters data are expressed as mean values ±standard deviation (n = 12—15). The mean grain size of the sediment samples was calculated according to the classification
of Wentworth (1922) and the equation of Folk and Wa rd (1957) . Significant differences between means are marked with different letters (p ≤0.05).
Station Latitude Longitude Date Depth
(m)
Water
temperature ( °C)
Salinity (S
A
) Mean grain
size ( μm)
WC (% of FW) OM (mg g
−1
DW) C/N ratio
(mol/mol)
S21 N 54 °13.290
E 12 °9.051
Apr 17 5.6 8.9 9.6 123 18.4 ±0.5
a 5.1 ±1.1
a 13 ±9
a
Jun 17 5.6 17.6 11.3 16.1 ±0.3
a 6.1 ±0.4
a 18 ±7
a
Jul 17 5.6 17.4 7.2 16.8 ±0.3
a 7.4 ±0.5
b 86 ±24
b
S25 N 54 °13.006
E 12 °9.730
Apr 17 3.0 8.9 9.2 129 19.9 ±0.3
a 4.2 ±0.4
c 6 ±1
c
Jun 17 3.0 17.6 11.4 19.2 ±0.4
a 4.6 ±0.7
c 8 ±2
c
Jul 17 3.0 17.4 7.6 18.9 ±0.8
a 4.1 ±0.2
c 4 ±1
d
S41 N 54 °14.002
E 12 °9.675
Apr 17 6.2 8.9 9.0 289 13.9 ±4.3
b 8.3 ±0.1
b 14 ±4
a
Jun 17 6.2 17.6 11.5 18.6 ±0.5
a 6.7 ±1.1
a 6 ±1
c
Jul 17 6.2 17.4 7.6 18.3 ±1.3
a 9.2 ±1.0
b 42 ±19
d
S45 N 54 °13.717
E 12 °10.354
Apr 17 4.4 8.9 9.3 131 19.4 ±0.3
a 6.0 ±0.9
a 9 ±2
a
Jun 17 4.4 17.6 11.3 17.5 ±0.4
a 5.4 ±0.2
a 13 ±4
a
Jul 17 4.4 17.4 7.7 17.0 ±0.5
a 4.1 ±0.4
c 4 ±1
d
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Figure 1 Location of the four sampling stations S21, S25, S41
and S45 in front of the site Hütelmoor at the southern Baltic
Sea coast, Germany. The isobaths for 3 and 5 m water depth
are shown as well. The frame represents the area of a detailed
bathymetric study of Kreuzburg et al. (2018) .
pling stations. The Warnow has a length of 143 km and an 156
elevation difference from spring to the mouth of 68 m. The 157
river mouth is located 15 km landwards where a weir pre- 158
vents sea water to travel further upstream. The mean out- 159
flow rate is 16.5 m
3 s
−1 with a mean nitrate concentration 160
of 1.78 mg l
−1
. During the passage of the Warnow plume 161
through the estuary, inorganic nutrients are entirely con- 162
sumed or mixed so that only recalcitrant substances like 163
dissolved organic material of the plume are able to reach 164
the study site off the Hütelmoor ( Jurasinski et al., 2018 ). 165
2.2. Field sampling 166
Sediment samples were taken by scientific SCUBA divers
167
along the depth gradient (see Figure 1 and Tab l e 1 ) at three 168
sampling dates in April, June and July 2017. Temperature 169
and salinity were measured in the surface water over the 170
sampling locations (HQ 40d multi, Hach Lange, Düsseldorf, 171
Germany). In order to get undisturbed sediment samples at 172
each sampling location Plexiglas® core liners (height 250 173
mm, inner Ø50 mm) were pushed into the sediment and 174
sealed with a rubber plug on the top. The tubes were then 175
pulled out and bottom-sealed with a second rubber plug 176
(always 3 replicates to account for heterogeneity). After- 177
wards, the sediment cores were transported as fast as pos- 178
sible under undisturbed and dark conditions to the labora- 179
tory. Here the samples were kept at 5 or 20 °C, respectively, 180
depending on the measured in situ temperature ( Ta b l e 1 ). 181
In order to avoid anoxic conditions in the core (due to el- 182
evated oxygen depletion in the dark) the overlaying water 183
columns on top were mildly bubbled with air prior further 184
processing. 185
2.3. Biomass determination and sediment 186
characteristics 187
As proxy for microphytobenthic biomass mg chlorophyll a 188
per m
2
was estimated for each sample. Always the top first 189
cm layer of each sediment core was taken with a scraper, 190
well mixed in a Petri-dish and divided into two subsamples. 191
One subsample was used for the extraction of chlorophyll 192
a and the other one was used for the determination of or- 193
ganic content, water content and C:N ratio. For chlorophyll 194
a analysis 1.13 cm
3
sediment was mortared and mixed with 195
3 ml of cold 96% ethanol (v/v) plus a spoon tip of MgCO
3
and 196
left overnight. Afterwards the suspension was centrifuged 197
at 6,240 x g for 5 min at 5 °C. The centrifuged pellets were 198
extracted again with ethanol, but this time incubated for 30 199
min to guarantee complete extraction of chlorophyll a . The 200
supernatants were photometrically measured (UV-2401PC, 201
Shimadzu) at wavelength 665 nm for chlorophyll a and at 202
750 nm for turbidity. The chlorophyll a concentration was 203
calculated according to HELCOM protocol (2015) and val- 204
ues of both extractions were summed up. Always 3 replicate 205
samples were used. 206
Water content (% fresh weight) of sediment cores was 207
determined by relative weight loss after drying a defined 208
amount (approx. 10 g) of sediment for 12—24 h at 105 °C. 209
In order to determine the organic content (OC) (% dry 210
weight) the dried sediment was combusted at 550 °C for 211
4h. For the analysis of particulate organic carbon and ni- 212
trogen (POC:PON ratio) between 200 and 250 mg dry sedi- 213
ment were homogenized using a mortar, weighted using an 214
analytical scale (accuracy 0.05 mg) (Sartorius MC210P, Göt- 215
tingen, Germany) and wrapped in silver foil for a treatment 216
with 50 μl 10% hydrochloric acid (v/v) to remove inorganic 217
carbon. After drying, the sample was packed air-tight in tin 218
foil and combusted in an element analyzer (Vario EL III, El- 219
ementar, Langenselbold, Germany). Grain size analysis was 220
conducted with a particle size analyzer (Type 1180, Cilas 221
Ltd., Orléans, France). Prior to the analysis small amount 222
of sediment (tea spoon) was dispersed in deionized water 223
and homogenized by sonication for 30 min. For each sample 224
sediment grains were split up into 100 size classes (0.37 up 225
to 2000 μm) in 12 replicates. Size information was summed 226
up to six different size classes ( > 1000, 1000—500, 500—200, 227
200—100, 100—63 and < 63 μm) which were used to calcu- 228
late the mean grain size of the sediment samples according 229
to the classification of Wentworth (1922) and the formula 230
provided by Folk and Ward (1957) . 231
2.4. Microphytobenthic community production ( ex 232
situ ) 233
The production and consumption of oxygen was used as 234
proxy for microphytobenthic community production and 235
respiration, respectively. Always three replicate sediment 236
cores per station (inner Ø50 mm, height 250 mm, vol- 237
ume ∼500 cm
3
) with intact sediment surface were taken 238
in the field and measured simultaneously in the labora- 239
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Figure 2 Schematic overview of the experimental setup. Sediment cores were placed inside a plastic tray. Water bath, perma-
nently cooled by a flow-through thermostat (arrows indicate flow direction) (1) and light source (daylight white LEDs) darkened
with shading foil to induce different photon fluence rates (2). Light came only from the top. Three sediment cores with mounted
measuring module (A—C) equipped with magnetic stirrer and fluorescent sensor spot (3) are connected to a control unit (4) via
optical fiber. A fourth dummy core filled with in situ surface water (D) is used for temperature measurement and compensation
during the experiment. MPB: Microphytobenthic biofilm on top of the sediment.
tory. Approx. 1/3 of the tube was filled with sediment and 240
2/3 was filled with overlaying water. The experimental de- 241
sign was similar to the setup published by Woelfel et al. 242
(2010) , but with various technical improvements and modi- 243
fications ( Figure 2 ). A self-constructed “measuring module”244
was mounted air tight on top of each core without disturb- 245
ing the sediment surface. This module was equipped with 246
a stirrer (Ø 30 mm, 6—7 rpm) powered by an external ro- 247
tating magnet, a valve and a commercial contactless oxy- 248
gen sensor spot (PyroScience, Ø5 mm, optical isolation). 249
The planar oxygen sensor spots were glued inside the mea- 250
suring modules and connected across the transparent Plex- 251
iglass housing to external optical fibers which transmitted 252
the light signals between the sensor spots and a 4-channel 253
control unit (FireSting O
2
, PyroScience, Aachen, Germany). 254
Calibration and measurements were controlled and logged 255
with the software Pyro Oxygen Logger version 3.213 sup- 256
ported by PyroScience. Before measurements, a two-point 257
calibration (0 and 100% oxygen saturation) was carried out 258
using filtered Baltic Sea water from the sampling location. A 259
rubber plug at the bottom of the sediment corer, adjustable 260
for height and a valve on top of the module were used to 261
remove air bubbles from the incubation room and to adjust 262
the incubated water volume above the sediment surface. 263
The latter was determined for each core and integrated in 264
the calculations. 265
Light was provided by LEDs (Seoul W42182-05LF, daylight 266
white) covering the PAR (photosynthetically active radia- 267
tion) range of 400-700 nm. LEDs were mounted directly on 268
top of the incubation tubes. The possible maximum pho- 269
ton fluence rate applied in this approach was approx. 800 270
μmol photons m
−2 s
−1
. Each core was irradiated by an in- 271
dividual light source. A fourth tube was filled with fresh in 272
situ water sampled on the same day from the same loca- 273
tion as the sediment cores for temperature measurements 274
during the experiment and used as signal for the tempera- 275
ture compensation in the optode software. All cores were 276
placed in a plastic tray (35 ×35 ×53 cm) which was cooled 277
to the measured in situ temperature with a thermostat (Ti- 278
tan 250, Aqua Medic, Bissendorf, Germany). Prior to each 279
measurement, sediment cores were kept for at least 30 min 280
in the water bath at the respective in situ temperature. Af- 281
terwards respiratory oxygen consumption in the dark and 282
photosynthetic oxygen production of the microphytoben- 283
thic communities with increasing photon fluence rates were 284
recorded in the original water volume above the sediment 285
surface of the core. The water column above the sediment 286
contained always < 2 μg chlorophyll a L
−1 which is con- 287
sidered as oligotrophic ( Håkanson, 2008 ), and hence phy- 288
toplankton was neglectable. Benthic diatoms were exposed 289
to 5 to 7 increasing light levels ranging from 0 to 750 μmol 290
photons m
−2
s
−1
of PAR . Photon fluence rates were measured 291
with a cosine corrected 2 πlight sensor (light meter LI-250, 292
LI-COR, Lincoln, United States of America) placed directly 293
next to the core on the same height as the sediment sur- 294
face. Measurements started with a respiration phase of 30 295
min in the dark followed by a 20 min photosynthesis phase 296
for each light level. The experiment was finished by a fi- 297
nal respiration phase for 30 min. Different light levels were 298
achieved by covering the LEDs with combinations of neutral 299
density filter foils. The distance from light source to the sed- 300
iment surface was kept constant during the measurement. 301
After the experiment the top first cm of each sediment core 302
was harvested and used for chlorophyll a determination as 303
described in detail above. The oxygen consumption and pro- 304
duction per time unit was referenced to the surface area 305
(mg O
2 m
−2 h
−1
). The resulting photosynthesis irradiance 306
(PI) curve data were fitted to the nonlinear model of Webb 307
et al. (1974) which describes the change in gross production 308
(GPP) with increasing photon fluence rate without photoin- 309
hibition: 310
GP P
(
P F D
)
= NP P
max
·1 −e
−α·PF D
NP P
max
+ R
with NPP
max as light saturated net production, αas the 311
slope of net production increase during initial photon flu- 312
ence rates (light limiting range), PFD as photon fluence rate 313
and R as dark respiration. Fitting of the data was conducted 314
with the Excel add-in Solver (MS Office 2013, Microsoft Co- 315
operation). 316
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2.5. Scanning electron microscopical investigation 317
Sand grains with attached diatoms from the sediment cores 318
were visualized and photographed by a field emission scan- 319
ning electron microscopy (SEM) operated at 5 kV (FE-SEM, 320
MERLIN® VP Compact, Carl Zeiss, Oberkochen Germany, 321
Faculty for Medicine, University of Rostock). Individual sand 322
grains were picked and washed gently in ultrapure water 323
several times to remove salt and other small particles. The 324
so prepared sand particles were mounted onto Aluminium 325
SEM-carriers with adhesive conductive carbon tape (Co. 326
PLANO, Wetzlar, Germany), coated with carbon (5—10 nm 327
layer) and exposed to a vacuum (EM SCD 500, Co. BALTEC, 328
Balzers, Liechtenstein). 329
From the shallowest station 25 one sediment core was 330
used for getting a crude overview on benthic diatom abun- 331
dances according the methodological approach of Woelfel 332
et al. (2010) . Species were morphologically identified using 333
taxonomic literature given by Woelfel et al. (2014a) . 334
2.6. Statistics 335
Statistical significance of the mean values of the respec- 336
tive abiotic data, chlorophyll values, net primary produc- 337
tion and respiration rates were tested with one-way ANOVA, 338
followed by a Tuk e y ’s multiple comparison test (P < 0.05). 339
Prior to this, a test for equality of variances ( Levene et al., 340
1960 ) and a normality test ( Shapiro and Wilk, 1965 ) was 341
conducted. Analyses were performed with InStat (GraphPad 342
Software Inc., La Jolla, California, USA). Photosynthesis ver- 343
sus photon fluence rates and respiration rates were fitted 344
to the model of Webb et al. (1974) using the Excel module 345
Solver. 346
3. Results 347
3.1. Environmental settings of both study sites 348
The Hütelmoor sampling stations S21, S25, S41 and S45 are 349
brackish with fluctuating salinities between 7.2 and 11.5 S
A
350
(absolute salinity) and temperatures ranging from 8.9 °C to 351
17.6 °C in April to July 2017 ( Tab l e 1 ) due to irregular up- 352
welling of cool, saline deep water at the outer Baltic Sea 353
coast ( Jurasinski et al., 2018 ). 354
The mean grain size of the sediment particles was very 355
similar at the study site, ranging from 123 to 289 μm (fine 356
to medium sand particles). 357
The water content of all sediment samples was very sim- 358
ilar with an average value of 18% of fresh weight ( Ta b l e 1 ). 359
The organic matter (OM) content of the sediments ranged 360
from 4.1 to 9.2 mg g
−1 dry weight, and the respective C/N 361
ratio (mol/mol) varied between 4 and 86 ( Ta b le 1 ). Partic- 362
ularly at both deeper stations (5.6 and 6.2 m) C/N ratio in- 363
creased 3 to 6-fold from April/June to July 2017 ( Tab l e 1 ). 364
3.2. Microphytobenthic standing stock biomass 365
The areal chlorophyll a concentration was determined as 366
widely used proxy for phototrophic biomass in all sediment 367
samples. A distinct depth gradient with significantly lower 368
Figure 3 Areal chlorophyll a concentration as proxy for pho-
totrophic biomass (chlorophyll a mg m
−2
) in all sediment sam-
ples, as measured in different water depths (m) and at three
sampling dates in April, June and July 2017. All values represent
the mean values with standard deviation (n = 12—15). Different
letters indicate significantly different means (Tukey’s test, P <
0.05).
chlorophyll a values was observed ranging from 28.5 mg Chl. 369
a m
−2
at 3 m depth to 87.7 mg Chl. a m
−2
at 6.2 m depth (p 370
< 0.05, Figure 3 ). Both shallow water stations at 3.0 and 4.4 371
m depth exhibited very similar chlorophyll a concentrations 372
(28.0 to 35.8 mg Chl. a m
−2
) in April, June and July 2017 373
( Figure 3 ). In contrast, at both deeper stations the chloro- 374
phyll a values were always higher, but also more variable. 375
Particularly the July sample at 5.6 m exhibited a strong de- 376
cline in chlorophyll a concentration from 71.2—85.2 mg Chl. 377
a m
−2
in April/June to 38.3 mg Chl. a m
−2
( Figure 3 ). 378
3.3. Microphytobenthic primary production and 379
respiration 380
Net primary production strongly varied across the sampling 381
dates and along the depth gradient from 3 to 6.2 m, ranging 382
from 29.4 to 178.9 mg O
2
m
−2
h
−1
( Figure 4 ). At the shallow- 383
est station (3.0 m water depth) net primary production was 384
the highest in April 2017 (129.9 mg O
2
m
−2
h
−1
) and the low- 385
est in June 2017 (39.7 mg O
2 m
−2 h
−1
), while in July 2017 386
an intermediate rate was measured (68.8 mg O
2 m
−2 h
−1
) 387
( Figure 4 ). A similar pattern could be determined at the 5.6 388
m station, where in April and June 2017 the highest pro- 389
duction rates were estimated (178.8 and 151.0 mg O
2 m
−2 390
h
−1
, respectively), followed by a sharp decline to 29.4 mg 391
O
2 m
−2 h
−1 in July 2017 ( Figure 4 ). At the deepest station 392
(6.2 m) net primary production was more similar across the 393
sampling dates, ranging from 68.5 to 104.6 mg O
2 m
−2 h
−1 394
( Figure 4 ). 395
In contrast to net primary production rates, respiration 396
rates were more similar at all stations and sampling dates. 397
The respiration values ranged from -9.9 to -20.8 mg O
2
m
−2 398
h
−1 ( Figure 5 ). The measured respiratory rates were low 399
since at each depth and sampling date net primary produc- 400
6
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Figure 4 Rates of ex situ net primary production expressed
as mg O
2 m
−2 h
−1 along the depth gradient from 3.0 to 6.2 m
and at three sampling dates in April, June and July 2017. All
values represent mean values with standard deviation (n = 3).
Different letters indicate significantly different means (Tukey’s
test, P < 0.05). n.d.: not detected.
Figure 5 Rates of ex situ respiratory oxygen consumption ex-
pressed as mg O
2
m
−2
h
−1
along the depth gradient from 3.0 to
6.2 m and at three sampling dates in April, June and July 2017.
All values represent the mean values with standard deviation
(n = 3). Different letters indicate significantly different means
(Tukey’s test, P < 0.05). n.d.: not detected.
tion rates always exceeded respiratory oxygen consumption 401
rates by a factor of 2.1 to 9.7. 402
From these oxygen measures gross primary production 403
(mg C m
−2 h
−1
) was calculated, by converting the O
2 val- 404
ues into C using a photosynthetic quotient (PQ = O
2
/ C) 405
of 1 ( Hargrave et al., 1983 ). For the study site next to the 406
Hütelmoor along the depth gradient from 3.0 to 6.2 m and 407
across the sampling dates from April to July 2017 a gross 408
primary production of 16.3 to 74.0 mg C m
−2
h
−1
was calcu- 409
lated ( Ta b l e 2 ). 410
3.4. Diatoms colonizing sediment particles 411
Benthic diatoms were attached to the sandy sediment par- 412
ticles collected at the exposed sites. Sand grains gener- 413
ally exhibited an intermediate sphericity with a smooth and 414
rounded surface without sharp edges, and they were inhab- 415
ited by only few diatoms of ca. 10 μm length, with Plan- 416
othidium delicatulum as most abundant species (25% of the 417
community). They preferentially occurred in some cracks 418
and crevices ( Figure 6 b, c and d). Many cells belong to ara- 419
phid taxa, hence cannot move and rather stick to the sur- 420
face by mucus coming from special pores at the cell poles. 421
4. Discussion 422
This is the first study in which primary production of mi- 423
crophytobenthic communities were measured in intact sed- 424
iment cores taken at different depths and sampling dates at 425
a sandy coastal site of the southern Baltic Sea using oxygen 426
planar optodes. The sediment cores were incubated under 427
controlled and undisturbed conditions in the laboratory to 428
evaluate potential maximum primary production. This, how- 429
ever, does not necessarily reflect in-situ conditions, since 430
the study site can be characterized as exposed location with 431
strong meteorological and hydrodynamic effects on shal- 432
low water microphytobenthic communities. Wind-induced 433
waves and currents along with water-level changes shape 434
the sediment properties, i.e. the sand grains are highly mo- 435
bile and hence rounded, both preventing attachment of en- 436
hanced cell numbers of benthic diatoms. This is well re- 437
flected in the low chlorophyll a values at 3 and 4.2 m depth 438
( Figure 3 ) and the generally low organic content of these 439
sediments ( Tab l e 1 ). At greater depth (5.6—6.2 m) the in- 440
fluence of waves and currents are dampened and hence 441
the sediments are less erosive resulting in doubled chloro- 442
phyll a concentrations compared to the shallower stations 443
( Figure 3 ) ( Ubertini et al., 2012 ; Van der Wal et al., 2010 ). 444
The prevailing wind direction and speed have been docu- 445
mented for all seasons for the wind flat system Bock (Zingst 446
Peninsula, German Baltic Sea coast, 55 km east of Hütel- 447
moor), which temporarily induce even an irregular and un- 448
predictable pattern of emersion and flooding ( Karsten et al., 449
2012 ). These authors reported that over 2.5 years continu- 450
ous measurements frequent short time intervals of flooding 451
occurred up to 20 and 50 cm water height, particularly dur- 452
ing storm events (wind speed > 8 m s
−1
), with strong forcing 453
of sediment resuspension and erosion. In addition, at the 454
exposed site the temperature amplitude was rather small 455
(8.9—17.6 °C) compared to more sheltered sites, because 456
there is irregular upwelling at the open coast ( Jurasinski et 457
al., 2018 ; Lehmann and Myrberg, 2008 ). The surface water 458
at sheltered sites warms more quickly as there is less mix- 459
ing. Consequently, microphytobenthic communities experi- 460
ence more hydrodynamic stress and more unfavorable tem- 461
perature conditions at exposed sites like in front of Hütel- 462
moor compared to, for example, shallow coastal lagoons of 463
the southern Baltic Sea, such as the Darss Zingst Bodden 464
Chain ( Meyercordt and Meyer-Reil, 1999 ). Ve rti cal mixing 465
and wave action are key factors controlling benthic diatom 466
growth on an exposed beach ( Steele and Baird, 1968 ), and 467
wave-generated shear stress and turbulence can cause re- 468
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Table 2 Gross primary production (mg C m
−2
h
−1
) estimates and standing stock biomass expressed as chlorophyll a concen-
tration (mg m
−2
) of benthic microalgae for temperate regions of the North Sea and Baltic Sea as determined by
14
C fixation
or O
2
production. The O
2
values were converted into C using a photosynthetic quotient (PQ = O
2
/ C) of 1 ( Hargrave et al.,
1983 ).
Location GPS data Depth (m) Sediment mean
grain diameter
( μm)
Method GPP (mg C
m
−2
h
−1
)
Chl. a
content
(mg m
−2
)
Reference
Westerschelde,
The Netherlands
N 51 °26.5’
E 3 °57.9‘
intertidal 224—301
14
C 15—80 15—32 Barranguet et al.
(1998)
Wadden Sea,
Germany, two
locations
N 55 °00.8‘ E
8 °26.3‘
N 53 °44.1‘ E
7 °41.9‘
Intertidal 139—380 O
2 29—51 176—194 Billerbeck et al.
(2007)
Ems Dollard
Estuary, The
Netherlands
N 53 °22.4’
E 6 °54.1‘
intertidal 80—110
14
C 10—115 5—560 Colijn and de
Jonge (1984)
Coastal lagoon,
Southern Baltic
Sea, Germany
N 54 °32.3‘
E 13 °07.5‘
2.5 100—490 O
2 6—18 135—209 Meyercordt and
Meyer-Reil (1999)
Sandy beach,
Southern Baltic
Sea, Poland
N 54 °27.1’
E 18 °34.1’
< 0.5 1,120—1,310 O
2 0.2—41.8 20—122 Urban-Malinga and
Wiktor (2003)
Laholm Bay,
Kattegat, Sweden
N 56 °35.1’
E 12 °50.0’
2—20 muddy sand
14
C 1—17.3 1—87 Sundbäck and
Jönsson (1988)
Gulf of Riga, Baltic
Sea, Estonia and
Latvia
N 58 °21.0’
to N
57 °15.0’
E 22 °10.0’
to E
24 °24.0’
0.2—5 Muddy sand
14
C 0.1—2.8 15—66 Vilbaste et al.
(2000)
Ythan Estuary,
Scotland, UK
N 57 °19.9’
W 1 °59
9’
< 1 336
14
C 9—226 163—221 Leach (1970)
Lynher Estuary,
England, UK
N 50 °21.9
W 4 °12.2’
< 2 < 63
14
C 163—523 5—115 Joint (1978)
Southern Baltic
Sea, Germany
see Tab l e 1 3—6.2 123—289 O
2 16.3—74.0 28.0—87.7 This study
suspension of surface sediments and their associated organ-
469
isms resulting in lower microalgal biomass ( de Jonge and van 470
Beusekom, 1995 ). 471
The sediment organic matter (OM) content ranged be- 472
tween 4 and 9 mg g
−1
dry weight across all stations and sam- 473
pling dates which is similarly low to 3—6 mg g
−1
dry weight 474
measured in a nearby wind flat at a station with very low 475
numbers of phototrophic microorganisms ( Woelfel et al., 476
2007 ). Assemblages of benthic diatoms and cyanobacteria, 477
however, increase the OM values in the wind flat to 12—17 478
mg g
−1
dry weight ( Woelfel et al., 2007 ). 479
The C/N ratios measured in April and June 2017 480
amounted between 6 and 18 across all stations, and are in 481
the same range as previously published ratios (10—12) for 482
sediments sampled in a shallow coastal lagoon at Hog Island 483
Bay, Virginia, USA ( Hardison et al., 2013 ). In July 2017, a 484
strong change in the C/N ratios could be observed. While 485
at both deeper stations the C/N ratios increased to 42—486
86, at both shallower stations the opposite was observed 487
with a strong decline of the C/N ratio to 4. High C/N ra- 488
tios generally indicate carbon-rich organic matter, which is 489
relatively poor in nitrogen. The study site is connected to 490
a coastal peatland, which stretches into the shallow wa- 491
ter zone ( Kreuzburg et al., 2018 ). These authors reported 492
also organic-rich layers further offshore with C/N ratios 493
< 71, which are similar to the data of the present study. 494
The submerged peat and organic-rich layers at the study 495
site are not always exposed to the water column, depend- 496
ing on wave-induced sediment movement which results in 497
erosion or burial ( Jurasinski et al., 2018 ). C/N ratios of 4 498
are rather unusual, but have been described for unialgal 499
cultures ( Falkowski and Owens, 1980 ) and for early spring 500
cyanobacterial plankton in the Baltic Sea ( Walve and Lars- 501
son, 2010 ). A cyanobacterial phytoplankton bloom did not 502
happen in July 2017 in front of Hütelmoor and hence can 503
be neglected as cause for the low C/N ratio in the shal- 504
low water stations. There is, however, significant subma- 505
rine groundwater discharge with dissolved nitrate from the 506
peatland into the nearshore Baltic Sea sediments ( Jurasinski 507
et al., 2018 ), which might explain the temporarily reduced 508
C/N ratios. 509
The underwater light field along the depth gradient is 510
complex because of the attenuation of incident solar ra- 511
diation due to the optical properties of the water column 512
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Figure 6 Representative habitat picture of the exposed sampling site in front of the Hütelmoor on 18
th
February 2016 (A). This
site is a near shore sandy beach, which is strongly influenced by westerly winds and a coastal parallel east-west current leading
to high hydrodynamic, erosive forces. Scanning electron microscopic pictures of sand grains with attached diatoms from the from
this exposed site (B, C, D). The sand grains are rounded with low numbers of attached, mainly araphid diatoms. and they occurred
mainly in cracks and crevices (C, D).
(yellow substances, re-suspended particles, phytoplankton 513
etc.). Maximum solar radiation during spring and summer 514
2017 ranged from approximately 1,900 to 2,100 μmol pho- 515
tons m
−2 s
−1 (own measurements). Ver tic al snapshot light 516
measurements at the deeper stations at noon in June and 517
July 2017 down to the seafloor resulted in calculated atten- 518
uation coefficients (K
d
) between 0.26 and 0.41 m
−1
. These 519
rather low K
d values point to a high to medium transmit- 520
tance of the water column in the PAR range, resulting in 135 521
to 157 μmol photons m
−2 s
−1 at noon at 6 m depth (own 522
measurements). Such PFDs are usually sufficient to satu- 523
rate the light requirements for photosynthesis in benthic di- 524
atoms. This is supported by measurements of the photosyn- 525
thetic performance of eight clonal benthic diatom strains, 526
which were isolated from the study site, and which exhib- 527
ited light saturation points between 32 and 151 μmol pho- 528
tons m
−2
s
−1
( Prelle et al., 2019 ). Similar low light require- 529
ments (34 to 100 μmol photons m
−2 s
−1
) were reported for 530
three benthic diatom species isolated from a sheltered sed- 531
iment of the southern Baltic Sea ( Woelfel et al., 2014a ). 532
Benthic diatoms are well known for their high photophysio- 533
logical plasticity and their capability for vertical movement 534
into or out of the sediment to avoid photodamage ( Ezequiel 535
et al., 2015 ). Physiological and behavioral photoprotection 536
are thus the two major mechanisms by which natural mi- 537
crophytobenthic communities protect themselves against 538
high incident solar radiation ( Cartaxana et al., 2011 ). They 539
are able to adjust their photosynthetic apparatus relatively 540
quickly to a new light regime ( Glud et al., 2002 ; Kühl et 541
al., 2001 ). The underlying mechanisms include, for exam- 542
ple, alterations of the size or composition of the photosyn- 543
thetic units with consequences for electron transport ca- 544
pacity ( Richardson et al., 1983 ). However, as mentioned be- 545
fore, most of the benthic diatoms observed were araphid 546
and epipsamic, and hence immobile. 547
As mentioned in the introduction microphytobenthic 548
biomass can strongly vary in space and time at all scales 549
on shallow coastal sediments ( Ubertini et al., 2012 ). There 550
are many reports indicating benthic diatom biomass changes 551
even on diurnal to bi-weekly tidal cycles and over the course 552
of the seasons ( Koh et al., 2007 ; Orvain et al., 2012 ; van 553
der Wal et al., 2010 ). Besides light, temperature and wind 554
conditions ( Ubertini et al., 2012 ), also nutrient concentra- 555
tions can control microphytobenthic biomass and primary 556
production ( Cibic et al., 2007 ), although sediment pore wa- 557
ter is generally considered as nutrient-enriched compared 558
to the overlying water column ( Garcia-Robledo et al., 2016 ; 559
Sundbäck et al., 1991 ). In addition, sediment grain-size is 560
not homogenous within tide-influenced coastal areas, lead- 561
ing to differential distribution of particle size-groups with 562
different degrees of erodibility which of course affects 563
benthic diatom biomass and activity ( Orvain et al., 2012 ; 564
Ubertini et al., 2012 ). The sediment types determine the 565
preferential occurrence of epipsamic or epipelic benthic di- 566
atoms causing related variation in their biomass ( Ubertini et 567
al., 2012 ). Microphytobenthos have mostly been studied in 568
temperate tidal-influenced estuaries ( Serôdio et al., 2020 ), 569
while for the Baltic Sea with its specific hydrographic and 570
environmental conditions only few data exist. 571
The used ex-situ planar oxygen optode approach, i.e. 572
to sample intact sediment cores as intact microecosystem 573
from the field and to undertake incubations under con- 574
trolled and simulated conditions in the laboratory, opens 575
many possibilities to estimate benthic production rates un- 576
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der various environmental settings. The advantage of this 577
set-up is the possibility to measure community respiration 578
and net production from many replicate sediment samples 579
with a defined area without subsampling or destructing. The 580
disadvantage of the core incubations is the fact that al- 581
though they well integrate total activity of the whole sed- 582
iment dwelling community they provide only a limited in- 583
sight into their vertical distribution and activity. It should 584
be mentioned, however, that during the laboratory incu- 585
bations, the impact of shear stress and turbulence induced 586
sediment resuspension, which frequently occur at the nat- 587
ural site, was ignored, and hence the productivity data re- 588
flect only the potential optimum under calm in-situ condi- 589
tions. In addition, to overcome the problem with the lim- 590
ited vertical distribution and activity, the best methodolog- 591
ical approach is the application of vertical oxygen micro- 592
electrode profiling, which, although time-consuming, al- 593
lows a three-dimensional determination of oxygen produc- 594
tion and consumption at any given depth point in the sed- 595
iment (e.g. Glud et al., 2002 ; Kühl et al., 1996 ; Revsbech 596
and Jorgensen, 1983 ). However, the extrapolation of such 597
fine-scaled results on a mm scale to larger sediment ar- 598
eas might be challenging because of sediment heterogene- 599
ity and biomass patchiness. Our ex-situ benthic incubation 600
set-up might be a feasible compromise as it integrates the 601
patchiness of microorganisms corresponding to the covered 602
sediment area of c. 20 cm
2
. In addition and in contrast to 603
the microsensor approach, the respective biomass param- 604
eter (chlorophyll a per sediment area) can be easily es- 605
timated, and thus a comparison with other studies facili- 606
tated. 607
The measured chlorophyll a concentrations at the sam- 608
pling site (28.0—87.7 mg m
−2
) was similar to those of 609
benthic microalgae from coastal temperate regions of the 610
North Sea and Baltic Sea ( Tab l e 2 ). Meyercordt and Meyer- 611
Reil (1999) and Urban-Malinga and Wiktor (2003) reported 612
chlorophyll a values of 135—209 and 20—122 mg m
−2
, re- 613
spectively, from sediment samples taken further east in a 614
coastal lagoon, southern Baltic Sea, Germany, and from a 615
sandy beach, southern Baltic Sea, Poland. The conspicuous 616
decline in chlorophyll a concentration from April/June to 617
July 2017 at the 5.6 m station compared to the unchanged 618
values at 6.2 m station can be explained by patchy distribu- 619
tion of microphytobenthic communities and different pre- 620
vailing physical forces affecting sediment structure. Both 621
sampling points are < 1.3 km distant, and hence it might be 622
possible that at 5.6 m sediments were more disturbed due 623
to wind and waves. 624
The respiration rates measured across all sampling sta- 625
tions and dates amounted -9.9 to -20.8 mg O
2 m
−2 h
−1
, 626
and hence were less variable compared to the net primary 627
production rates. While Sundbäck et al. (1991) determined 628
very similar respiration rates up to -19.5 mg O
2 m
−2 h
−1
, 629
other authors reported much higher values (-48.6 mg O
2
m
−2
630
h
−1
, Urban-Malinga and Wiktor, 2003 ; -46.0 mg O
2
m
−2
h
−1
, 631
Woelfel et al., 2010 ; -37.2 mg O
2
m
−2
h
−1
, Yap, 1991 ). Ben- 632
thic respiration strongly depends on OM content as substrate 633
for bacterial mineralization in combination with high abun- 634
dances of micro- and meiofauna ( Ya p , 1991 ). The rather low 635
OM values of the Hütelmoor stations together with the reg- 636
ular sediment disturbance would reduce such heterotrophic 637
activities. 638
The measured O
2 values were converted into C equiv- 639
alents using a photosynthetic quotient (PQ = O
2
/ C) of 640
1 ( Hargrave et al., 1983 ), although other authors used PQ 641
values between 0.9 and 1.3 depending on light and nutri- 642
ent availability ( Cahoon and Cooke, 1992 ; Glud et al., 2009 ; 643
Ni Longphuirt et al., 2007 ). Based on our approach we cal- 644
culated gross primary production of 16.3—74.0 mg C m
−2 645
h
−1
along the depth gradient and across the sampling dates 646
( Tab l e 2 ). These values are higher compared to sediments 647
of a coastal lagoon, southern Baltic Sea, Germany (6—18 mg 648
C m
−2 h
−1
, Meyercordt and Meyer-Reil, 1999 ), to those of 649
a sandy beach, southern Baltic Sea, Poland (0.2—41.8 mg 650
C m
−2 h
−1
, Urban-Malinga and Wiktor, 2003 ) and to those 651
in the Gulf of Riga (0.1—2.8 mg C m
−2 h
−1
, Villabaste et 652
al., 2000), but similar to other locations in the Kattegat and 653
Wadden Sea ( Ta b l e 2 ). 654
For the Baltic Sea a total C budget based on all primary 655
producers is still missing, because microphytobenthic com- 656
munities have been largely ignored so far and because the 657
pelagial is considered as the main compartment for C fix- 658
ation ( Schiewer, 2008 ). However, coastal areas are consid- 659
ered among the most productive ecosystems worldwide, and 660
here a combination of benthic and pelagic habitats con- 661
tribute to total primary production ( Ask et al., 2016 , and 662
references therein). Benthic diatoms can be responsible for 663
up to 20% of the ocean gross primary production although 664
occupying only 0.03% of the ocean surface area, i.e. shal- 665
low coastal regions ( Pinckney, 2018 ). Compared to the wa- 666
ter column, sediments are typically enriched in pore water 667
nutrients and if sufficient light is available, such coastal soft 668
bottom ecosystems are often dominated by benthic primary 669
production from polar to tropical regions ( Cahoon, 1999 ; 670
Glud et al., 2002 , 2009). The nearest data set to the Hütel- 671
moor of the southern Baltic Sea is from the Gulf of Gda
´
nsk, 672
where the average primary production of the phytoplank- 673
ton comprises 3.3 mg C mg Chl. a
−1
h
−1
( Renk and Ochocki, 674
1998 ) and that of the microphytobenthos around 1 mg C 675
mg Chl. a
−1 h
−1 ( Urban-Malinga and Wiktor, 2003 ), i.e. 23% 676
of the total primary production originated from benthic di- 677
atoms. A recent study on the Bothnian Bay (Northern Baltic 678
Sea) reported similar values with a share of 31% of the to- 679
tal annual primary production by microphytobenthic com- 680
munities ( Ask et al., 2016 ), and these authors also pointed 681
to the lack of data regarding benthic primary production in 682
the Baltic Sea. 683
Compared to earlier studies (e.g. Woelfel et al., 2010 ) 684
the community respiration rates were always very low at the 685
study site, generally representing < 20% of the photosynthe- 686
sis signals ( Figure 5 ). Therefore, it is reasonable to assume 687
that the heterotrophic activity based on bacteria and meio- 688
fauna was strongly reduced, which is well reflected in the 689
low organic matter content, and can be explained by the 690
strong prevailing hydrodynamic forces. 691
We are aware that the data shown represent only a snap- 692
shot under optimal conditions, and hence it would be very 693
important to consider all seasons (as in Urban-Malinga and 694
Wiktor, 2003 ) with their strongly fluctuating environmental 695
conditions to better understand the in-situ net primary pro- 696
duction of shallow water benthic diatoms of the Baltic Sea. 697
A modelling approach, as reported for Arctic microphyto- 698
benthic primary production, might be highly useful to ad- 699
dress this task ( Woelfel et al., 2014b ). 700
10
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5. Conclusion 701
In conclusion, our hypothesis that with increasing water 702
depths the effect of strong wind- and wave-induced mix- 703
ing of the upper sediment layer are dampened and hence 704
support higher microphytobenthic biomass and primary pro- 705
duction could be confirmed. Consequently, microphytoben- 706
thic communities play an important ecological role at the 707
shallow southern Baltic Sea coast, and hence their contri- 708
bution to the total primary production should be much bet- 709
ter evaluated in space and time. Such data are important 710
for the calculation of a realistic complete Baltic Sea carbon 711
budget, as reported so far only for the Bothnian Bay ( Ask 712
et al., 2016 ). In addition, if the microphytobenthic primary 713
production contributes with one third to the total Baltic 714
Sea production, its biogeochemical fate under global change 715
scenarios should be much better evaluated because of fu- 716
ture weather and climate change scenarios for the southern 717
Baltic Sea region. Dryer summers with more frequent and 718
occasional extreme storm events are to be expected ( BACC 719
II Author Team, 2015 ), leading to higher disturbance of ex- 720
posed sandy sediments with negative effects on microphy- 721
tobenthic primary production. 722
Uncited References
Q2
723
HELCOM 2018 724
Declaration of Competing Interest 725
None 726
Acknowledgements 727
We gratefully thank Peter Feldens for the grain size deter- 728
mination, the crew of the r/v Elisabeth Mann Borgese and 729
the SCUBA diving team led by Gerd Niedzwiedz for techni- 730
cal support in the field. In addition, we thank Juliane Müller 731
for her support during sample processing and analysis. We 732
are grateful to the Electron Microscopy Center of the Uni- 733
versity of Rostock, which supported the scanning electron 734
microscopy. This study was conducted within the framework 735
of the Research Training Group Baltic TRANSCOAST funded 736
by the DFG ( Deutsche Forschungsgemeinschaft ) under grant 737
number GRK 2000/1 (Subproject B2: Microphytobenthos). 738
This is Baltic TRANSCOAST publication no. GRK2000/0038. 739
Supplementary materials 740
Supplementary material associated with this article can be 741
found, in the online version, at doi:10.1016/j.oceano.2021. 742
02.002 . 743
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