![Solute fluxes[mmol/m² day⁻¹] (in order from upper to lower panel: Oxygen, ammonium, phosphate, silicate) during night‐time (left column, grey background) and daytime (right column, yellow background) incubations of each sampling day. On day 1. ‘Acclimation’, when all chambers (n = 10) were at same conditions at about 18°C. After 6 days of heating the marine heatwaves (MHW) chambers (n = 5) to about 26°C ‘Induced MHW’ (2.), while control chambers (n = 5) were at about 21°C. After another 6 days, when all chambers were back to natural conditions at about 23°C during an ‘Ambient MHW’ (3.) together with a strong phytoplankton bloom.](https://www.researchgate.net/publication/390179418/figure/fig5/AS:11431281324382058@1742968154822/Solute-fluxesmmol-mday-in-order-from-upper-to-lower-panel-Oxygen-ammonium_Q320.jpg)
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Functional Ecology. 2025;00:1–13.
|
1wileyonlinelibrary.com/journal/fec
Received: 20 March 2024
|
Accepted: 5 February 2025
DOI : 10.1111/1365-2435.70028
RESEARCH ARTICLE
Marine Heatwaves
Marine heatwaves amplify benthic community metabolism and
solute flux in a seafloor heating experiment
Norman Göbeler | Laura Kauppi | Alf Norkko | Joanna Norkko
This is an op en access arti cle under the ter ms of the Creative Commons Attribution L icense, which pe rmits use, dis tribu tion and reprod uction in any med ium,
provide d the original wor k is properly cited.
© 2025 The Author (s). Functional Ecology published by John Wil ey & Sons Ltd on behalf of British Ecological Society.
Tvärminne Zoologic al Station, Universit y
of Helsinki, Hanko, Finlan d
Correspondence
Norman Göbeler
Email: norman.gobeler@helsinki.fi
Funding information
Walter ja An drée de Nottbeckin
Säätiö; HORIZON EURO PE Resea rch
Infras truc tures , Grant /Award Numb er:
730984
Handling Editor: Pol Capdevila
Abstract
1. The magnitude and frequency of marine heatwaves are increasing and predicted
to intensify, but our ability to understand the real- world effects on vital benthic
ecosystems is lagging behind. Prior insights into the impacts of marine heatwaves
are often derived from observational or laboratory studies. Observational stud-
ies may not fully disentangle the complexities of potential compound events and
typically focus on severe, often lethal marine heatwaves. Laboratory studies, on
the contrary, while valuable for understanding specific mechanisms, often use
artificial setups and can introduce unnatural disturbances that do not reflect real-
world scenarios.
2. To investigate sublethal temperature effects of marine heatwaves in a natural
benthic habit at, we developed a novel approach for inducing elevated water tem-
peratures in situ over several days. The system utilizes domestic underfloor heat-
ing technology combined with custom- made benthic chambers.
3. We placed 10 chambers for 15 days in a bare- sediment habitat at 2.5 m depth
and heated five chambers to 5°C above ambient water temperatures in summer
for 6 days, followed by a period of 7 days at ambient temperatures. Incubations
during day and night were performed during the experiment to assess changes
in ecosystem functioning (solute fluxes) and sediment cores were collected at
the end of the experiment to assess the effects of a realistic marine heatwave on
benthic community structure.
4. The results indicate that while the benthic community structure remained simi-
lar between the treatments, except for a size shift of Marenzelleria spp. towards
smaller individuals in the heated treatment, elevated temperatures caused a sig-
nificant increase in community respiration and amplified the magnitude of either
efflux or influx of nutrients (NH4
+- N, PO4
3−- P and Si).
5. Primary production during daytime incubations remained mostly unaffected by
the heatwave treatment, contributing to the concept of heterotrophy being more
influenced by increased temperatures than autotrophy.
6. This study confirms the suitability of the novel system for examining the impact
of temperature on benthic habitats in situ and demonstrates its potential for the
2
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GÖBELER et al .
1 | INTRODUC TION
Addressing the complexity of ecosystem responses to marine
heatwaves (MHWs) poses a significant challenge. The frequency,
duration and intensity of MHWs are predicted to increase in the
future (Frölicher et al., 2018) and already now the severe impacts
that even short- term temperature anomalies can have on marine
ecosystems have been highlighted. For example, MHWs have
caused mass mortalities around the globe (Cavole et al., 2016;
Garrabou et al., 2009, 2019; Genin et al., 2020; Rogers- Bennett
& Catton, 2019; Seuront et al., 2019), with a range of different
effects across multiple scales of biological organization (reviewed
in Smith et al., 2023).
MHWs at the seafloor may have significant impacts on ecosys-
tem processes, particularly because a majority of organisms inhab-
iting the seafloor are ectothermic and relatively sessile. Macrofauna
in soft sediments contribute directly and indirectly, through their
metabolism and activities in the form of bioturbation, to benthic-
pelagic coupling (Kristensen et al., 2012). The recycling of organic
material, either through the feeding of macrofauna or by modify-
ing the sediment matrix, enhancing the oxygen supply to sediment
microbial communities, is a key process in linking the benthos and
pelagos (Karlson et al., 2007). Macrofauna can, depending on the
environmental context, account for up to 69% of the variability in
solute fluxes (Gammal et al., 2019). On an archipelago scale, Gammal
et al. (2019) found temperature to be the most important environ-
mental variable contributing 6% of total variation in solute fluxes,
with the cumulative contribution of all environmental variables
being 20%. In this context, the role of MHWs may become partic-
ularly crucial. While the general importance of temperature for bio-
chemical processes is well known, only a few studies have thus far
investigated the effect s of episodic events, such as MHWs on nutri-
ent fluxes. In a laboratory study with intac t sediment cores from a
deeper, muddy site, Kauppi et al. (2023) found that repeated, strong
MHWs can increase bioturbation activities and thus enhance carbon
remineralization. While a similar effect was detected by Kauppi and
Villnäs (2022) also utilizing intact sediment cores from a muddy site,
this study also investigated even higher temperatures that resulted
in a severe reduction in oxygen consumption possibly related to the
inactivation of microbial processes. Another laboratory study by
Dolbeth et al. (2021) investigating estuarine benthic communities,
which normally experience broad ranges of temperatures, found
a slightly increasing trend in bioturbation and associated nutrient
releases but also highlighted a possible tolerance to MHWs as the
responses were inconsistent. Together, these studies highlight the
complexity of responses and the need for further studies to unravel
the effects of MHWs on seafloor ecosystem functioning.
Understanding natural community responses requires manip-
ulative experimentation on the scale of mesocosms where natural
communities are assembled, or in situ enclosures. The latter include
a high degree of biological complexity and environmental variability,
which are necessar y for an ecologically relevant response, but can
be a logistic and infrastructural challenge (Boyd et al., 2018; Gerhard
et al., 2023), particularly when involving heating. In benthic ecology,
laboratory studies inherently involve the collection of sediment,
which disturbs the sediment structure and any biogeochemical gra-
dients in the sediment, thus potentially affecting the results. Yet,
laboratory studies can generate valuable insights, particularly into
mechanistic effects (Boyd et al., 2018). In contrast, insights gained
from field obser vations on the effects of MHWs benefit from re-
alism, but (1) might only highlight extreme summer temperatures
with direct lethal effects possibly overseeing sublethal effects, (2)
often lack environmental and biological monitoring baseline data to
relate the effects to and (3) could misinterpret the cause- effect re-
lationships, as other factors can accompany elevated temperatures
and the effects could, in reality, be the results of a compound event
(Burger et al., 2022; Gruber et al., 2021). Hence, there is a trade- off
between experimental approaches in the laboratory that are limited
in their real- world realism and the observational approaches explor-
ing MHWs in situ that lack control. This trade- off highlights the need
for in situ experimental approaches as a necessar y way forward for
an improved mechanistic insight into MHW effects.
Currently, studies investigating the isolated effects of tempera-
ture in situ on benthic ecosystems are rare, likely due to the complex
logistics. The system applied by Egea et al. (2023), combining small
rigid cylinders with a gas- permeable cover and heaters connected to
a boat, to investigate the effec t of MHWs in different seasons on the
carbon metabolism of two macrophyte species was the most elabo-
rate one to date. Here, we used a unique newly developed system,
coupling large (50 cm diameter) benthic chambers with a domestic
house floor heating system (Göbeler et al., 2023), to induce a MHW
in situ and to investigate the response of a benthic community to the
MHW in summer in a shallow unvegetated sandy habitat. We inves-
tigated changes in community structure, in ecosystem functioning
(specifically nutrient fluxes) and communit y oxygen met abolism. In
addition to the induced MHW, the area experienced a natural MHW
right after the experiment had started, which further highlighted the
complexity of investigating the effects of MHWs in situ.
investigation of complex habitats and communities, which are essential for our
understanding of the ecosystem- level effects of climate change.
KEY WORDS
benthic ecology, benthic- pelagic coupling, climate change, community metabolism, in situ
study, marine heatwave, temperature ef fect
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GÖBELER et al .
2 | MATERIALS AND METHODS
2.1 | Replication statement
Scale of
inference
Scale at which the
factor of interest is
applied
Number of replicates at
the appropriate scale
Natural
Community
Chamber Five heated and five
control chambers
2.2 | Warming and marine heatwaves in the
study area
The Baltic Sea is a shallow, semi- enclosed sea where anthropogenic
influence and warming happen at an accelerated rate compared with
open oceans. It is characterized as a multistressor system, suffering
from, for example warming, eutrophication and expanding hypoxic
zones, but it is also rich in highly productive coastal areas (Reusch
et al., 2018). For example, in 2018, a MHW event in July/August
covered large parts of the Baltic Sea, causing a positive sea surface
temperature anomaly for the entire Baltic Sea of 4–5°C (Rutgersson
et al., 2022). This 2018 MHW event across the Baltic Sea also caused
the maximum bottom temperature ever recorded at a long- term
coastal monitoring site active since 1926 at the south coast of Finland,
near the location of the field experiment described here, with 21°C at
31 m depth (July 2018) (Goebeler et al., 2022). However, the highest
ever recorded temperature in the surface layer of the long- term moni-
toring site was in August 2003, with a maximum of 25.3°C.
2.3 | Experimental setup
The field experiment was conducted in a nature reserve owned by
the University of Helsinki, and permission for research was granted
by Tvärminne Zoological Station (University of Helsinki). The study
did not require any ethical approval.
The experiment was deployed and sampled using scuba diving. Ten
chambers were placed in July 2021 on a mostly unvegetated, sandy
seafloor, at a depth of about 2.5 m, in a semi- enclosed (protected
from prevailing SW- winds) bay of the island Furuskär (59°50′0.25″ N,
23°15′45.54″ E) in the south- west Finnish Archipelago. While few
single strands of vegetation were present at the site, the chambers
were positioned to avoid these areas. For a detailed technical descrip-
tion of the benthic chambers and heating system, please see Göbeler
et al. (2023). In short, the closed- circuit system uses an electric heater
to warm water, which is circulated through tubing coiled inside the
chamber walls and regulated by thermostats with sensors placed in
the centre of each chamber (Figure 1). Each chamber covered an area
of 0.19 m2 (inner diameter 49.2 cm) with a height above the sediment
surface of 15 cm (5 cm into the sediment) enclosing a water volume of
about 28.5 L. A hole of 5 cm diameter on the chamber wall, covered
with a 2 mm mesh, was always open except during incubations (see
section on solute fluxes) and ensured near- natural conditions inside
the chamber. Additionally, a circulation pump with a diffuser prevented
the build- up of physical and chemical gradients and accumulation of
metabolites. The chambers were deployed on 1 July 2021, with the lids
installed after a 24 h acclimation period and stayed in place until 15
July 2021. Five chambers served as control at ambient water tempera-
tures, while five chambers served as the MHW treatment (Figure 2).
The heating of the five treatment chambers started on 3 July. and
lasted for 6 days. The induced MHW was of ‘severe’ category (Hobday
et al., 2018) (25.7 ± 1.1°C averaged over the five MHW chambers
during the induced MHW period). Thereafter, the heating was termi-
nated, and the chambers were allowed to cool to the ambient tempera-
ture within 12 h and remained at ambient temperature until the end.
The five control chambers, built identically to the heated chambers, ex-
perienced a natural MHW starting on 2 July 2021 when the threshold
at about 18°C was exceeded and gradually increased to about 24°C
(corresponding to a ‘strong’ category) on 15 July 2021. The climatologi-
cal reference values are based on Goebeler et al. (2022) from the refer-
ence period 1931–2020 of the surface layer (0–2 m) from Storfjärden,
a monitoring site which is about 2.8 km from the experimental site. All
temperature values are daily averages unless otherwise mentioned.
2.4 | Solute fluxes and community metabolism
The gas- tight benthic chambers allow measurement of oxygen and
nutrient fluxes when interrupting the water exchange with a rubber
FIGURE 1 Overview of Hotfloor system from, the (a) surface
(drone picture by, Alf Norkko) and (b) underwater at the seafloor.
The temperature control unit, consisting of 10 thermostats and the
electric heater, is placed on shore ①. The electric heater is connected
to a closed water circuit distributing warmed water through about
40 m long tubing ② to the weighted benthic chambers placed on the
seafloor (b). About 10 m of tubing are coiled inside the chamber walls
③ to ensure a gentle heat exchange with the surrounding water. The
chamber lid is equipped with various ports for sampling, a circulatory
pump with diffuser ④ and sensors ⑤.
4
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GÖBELER et al .
plug (Göbeler et al., 2023). After the rubber plug was inserted, a
100 mL syringe was attached to the sampling port, and once the
neighbouring port was opened (ambient replacement water), the
Start water sample was withdrawn. After 4 h, the End sample was
taken in a similar manner, and the rubber plug was removed to
flush the chambers, returning to near- natural conditions. All water
samples were filtered through combusted GF/F Whatman filters
and stored at −20°C until laboratory analyses. Nutrients (NH4
+- N,
PO4
3−- P, Si) in the water samples were analysed with a nutrient au-
toanalyser (Aquakem 250; Thermo Scientific).
Oxygen concentrations and temperature near the sediment sur-
face were measured with HOBO Dissolved Oxygen Loggers (U26-
001, Onset Computer) hung above the sediment surface in the
centre of the chamber, logging at 15- min intervals throughout the
experiment. To estimate changes in the oxygen concentration during
the incubations, the last oxygen concentration data point before the
incubations started was chosen as the start concentration, and the
last data point before the incubation ended as the end concentration.
Knowing the water volume inside the chamber, the enclosed area,
incubation time and the start and end concentrations, the direction
and magnitude of nutrient and oxygen fluxes were calculated. All
fluxes are expressed as mean ± SD and in the unit mmol m−2 day−1.
In total, six incubations on three sampling days were conducted
during day time (start between 10:00 AM and 12:00 PM) and
night- time (start between 10:00 PM and 12:00 AM). These light and
dark incubations were used to assess the role of primary production
in the oxygen dynamics within the chambers. On the first sampling
day 2 July 2021 (‘Acclimation’), all 10 chambers were at ambient
temperature conditions (about 17.7 ± 0.6°C). After 6 days of heat-
ing five chambers to 25.7 ± 0.1°C, the second sampling (‘Induced
MHW’) was conduc ted. After another 6 days with all chambers at
ambient temperature conditions at about 24.2 ± 0.5°C, due to the
naturally occurring heatwave, the final sampling (‘Ambient MHW’)
was conducted.
2.5 | Macrofaunal community
Macrofauna samples were collected via scuba diving with a sedi-
ment corer (8.4 cm diameter, 10–15 cm depth) after the removal of
the chamber lid at the end of the experiment, as any coring during
the experiment would have severely disturbed the sediment. The
size of the corer to sample the macrofaunal community is a com-
promis e bet we en th e han dl in g of th e diver and a go od ex pl an atory
power when linking the community to the ecosystem func tioning
(Norkko et al., 2015). In this area, most macrofauna are found in
the top 3 cm of the cores; a few individuals are found down to
10 cm depth, but virtually none below that. Thus, 10–15 cm is a
sufficient coring depth to represent the infaunal community. The
sediment was sieved through a 0.5 mm mesh and stored in 70%
ethanol. The benthic community was identified under the micro-
scope to the lowest practical taxonomic level. Furthermore, abun-
da nce, bio ma ss (b l ot ted wet weig ht ) and bo d y size we re me a su red .
No si gns of mor tality, that is su rfaced animal s, were obser ve d du r-
ing the experiment.
2.6 | Statistical analyses
The abu nd an ce and biomas s of the macrof au na l community were in -
vestigated in a one- way permutational analysis of variance, based on
the Bray–Curtis dissimilarity index, to test for differences between
the communities by treatment (PERMANOVA; Anderson, 2001,
2017). To visually assess differences in the community struc ture be-
tween heated and control chambers, a non- metric multidimensional
scaling (nMDS) based on the Bray–Curtis dissimilarit y index was
calculated. Furthermore, we assessed the impact of treatment on
the size distribution of the most dominant (by biomass) bioturbators,
Macoma balthica, Hediste diversicolor and Marenzelleria spp. We per-
formed a species- specific one- way PERMANOVA to test the effect
of treatment on the sizes, based on Euclidean distance.
The investigation of the MHW effects on ecosystem function-
ing, that is changes in biogeochemical fluxes (O2, NH4
+- N , P O 4
3−- P
and Si), was conducted using PERMANOVA, base d on Euclid ean dis-
tances. Multivariate analysis in the form of PERMANOVA allows for
simultaneous analysis of multiple response variables, capturing the
multivariate nature of ecosystem processes. This approach considers
FIGURE 2 Overview of daily average temperature of induced
marine heatwaves (MHW) treatment and control at ambient
temperature (dashed line) and sampling occasions (numbers 1–3).
The line colours show the MHW categories (yellow = moderate,
orange = strong, red = severe) when exceeding reference values
and the blue line shows the climatological mean based on period
1931–2020 (Goebeler et al., 2022). The thresholds for the MHW
categories here are, for example on the 8 July 2021, for C ategory I
‘moderate’ 18.5°C, for Category II ‘strong’ 22.2°C, for Category III
‘severe’ 25.9°C . During 1—‘Acclimation’ all chambers (n = 10) were
at ambient temperatures of about 18°C. MHW treatment chambers
(n = 5) were heated to about 26°C until sampling 2—‘Induced MHW’,
while control chambers (n = 5) remained at ambient conditions of
about 21°C. Heating was terminated in the morning of 10 July 2021
and MHW chambers cooled down to ambient temperatures. During
the last sampling 3—‘Ambient MHW’, all chambers were at about
24°C—a naturally occurring strong MHW.
16
20
24
Jul-02 Jul-05 Jul-08
Jul-11
Jul-14
Date
Temperature [°C]
1
2
3
ambient temperature
induced MHW
|
5
GÖBELER et al .
the overall pattern of variation particularly useful for multiple, com-
plex response variables. However, as oxygen fluxes (primar y produc-
tion and respiration) are a strong proxy for ecosystem ac tivit y (Rodil
et al., 2020) and a strong driver in the multivariate space, we con-
ducted PERMANOVAs separately for oxygen and the combined nu-
trient fluxes (NH4
+- N , P O 4
3−- P, Si). In the first step, an overall model
(three- way PERMANOVA) testing the effects of Treatment, Day/
Night, Sampling and their interac tions with permutations constrained
by each chamber, to account for repeated measures over time, was
performed. Given that the treatment was only actively running in
two (‘induced MHW ’) out of six samplings, the overall model might
mask or underscore the effect of treatment. To specifically explore
the effect of treatment, a one- way PERMANOVA for each subset
of sampling and Day/Night was conducted. This approach accounts
for the greatly different natural conditions during the experiment,
that is increasing ambient temperatures and a strong phytoplankton
bloom during the last sampling. Additionally, the processes influenc-
ing the solute fluxes greatly differ when light is available compared
with dark conditions. The associated dispersion analysis (PERMDISP)
was used to confirm equal variances. To visualize the complex pat-
terns of the multivariate response for all solute fluxes, a principal
component analysis (PC A) was performed for the ‘Induced MHW’
sampling to facilitate the interpretation of the relative contribution
of oxygen, ammonium, silicate and phosphate. No transformations
were applied to the macrofauna community data, and all analyses
of solute fluxes were conducted after normalizing (Anderson, 2001)
and were conducted in R studio (version 2023.12.0). All analyses re-
garding the MHW, temperature values and climatological references
were based on the reference period 1931–2020 of the surface layer
(0–2 m) from Storfjärden (Goebeler et al., 2022) and were conducted
with the R- package heatwaveR (Schlegel & Smit, 2018).
3 | RESULTS
3.1 | Macrofaunal community
The experimental area was mostly unvegetated, except for small
single shoot s of Potamogeton perfoliatus, and the se diment was ch ar-
acterized by relatively low organic matter content (about 1%) and
a Mud (<63 μm) content of 15% ( Vesanen, 2023). The macrofaunal
communit y was comprised of benthic species typical for shallow
co as tal area s in th is pa r t of th e Balt ic Se a . Of th e tot al nine ta xa id e n-
tified, the bivalve Macoma balthica an d the pol ychae te s Marenzelleria
spp. and Hediste diversicolor were the most common with regard
to biomass. In terms of abundance, Oligochaet a, Ostracoda and
Marenzelleria spp. were the most prevalent. PERMANOVA con-
firmed the results of visual assessment of the nMDS plots (Figure 3)
that there were no significant differences between treatment s in
the community abundance (p = 0.44) or biomass (p = 0.12). MHW 2
and Control 1 in Figure 3b Biomass were likely more dissimilar to
the other sampling cores due to the presence of a few individual
large Cerastoderma glaucum, which were absent in all other sampling
cores. The visual inspection of the density plots of the size distri-
butions (Figure 4) of the most prominent bioturbators suggested a
higher frequency of smaller individuals in the heated treatment for
Macoma balthica and Marenzelleria spp. Specifically, Macoma balthica
of about 5 mm shell length and Marenzelleria spp. of about 0.6 mm
width for the MHW communities were the most common sizes. The
most common sizes in Control communities were 12 mm shell length
and 1.7 mm width for Macoma balthica and Marenzelleria spp., re-
spectively. However, the one- way PERMANOVA test for the effect
of treatment on the size distributions did not indicate a significant
difference for Macoma balthica (R2 = 0.06, pseudo- F = 2.14, p = 0.14)
and Hediste diversicolor (R2 = 0.01, pseudo- F = 0.19, p = 0.68), but
a significant effect on the size distribution of Marenzelleria spp.
(R2 = 0.14, pseudo- F = 12.81, p = 0.001). Specifically, in total, we
found 34 individuals of Marenzelleria spp . in all MHW cor es, of which
15 were larger than 0.6 mm, and in the Control, we found a total of
44 individuals, of which 34 were larger than 0.6 mm.
3.2 | Changes in ecosystem functioning
The ambient water temperature was about 18°C for all chambers
during ‘Acclimation’. During the ‘Induced MHW’ temperature had
naturally increased to 21°C in the Control chambers compared with
26°C in the five heated chambers. During the ‘Ambient MHW’, all 10
FIGURE 3 Non- metric multidimensional scaling (nMDS) of the
benthic communities based on abundance (panel a) and blotted
wet weight biomass (panel b). The Control and marine heat waves
(MHW) treatment s are labelled blue and orange, respectively.
PERMANOVA results indicate no significate differences bet ween
Control and MHW treatments for both abundance (p = 0.44) and
biomass (p = 0.12).
Control.1
Control.2
Control.3
Control.4
Control.5
MHW.1
MHW.2
MHW.3
MHW.4
MHW.5
Stress: 0.101
-0.4
-0.2
0.0
0.2
NMDS2
NMDS2
(a)
Abundance
(b)
Biomass
Control.1
Control.2
Control.3
Control.4
Control.5
MHW.1
MHW.2
MHW.3MHW.4
MHW.5
Stress: 0.038
-0.6
-0.3
0.0
0.3
0.6
-0.4 -0.2
0.00.2
-1.0 -0.5
0.0 0.5
NMDS1
6
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GÖBELER et al .
chambers were at 24°C. Additionally, during the last sampling, there
was a strong phytoplankton bloom observed in the water as well
as in all chambers. During ‘Acclimation’ and ‘Induced MHW’ mean
oxygen fluxes were negative both during Day and Night incubations,
but oxygen consumption was always greater during Night than dur-
ing Day (Table 1, Figure 5). During the ‘Ambient MHW’, however,
oxygen was produced during the Day but consumed at Night. Higher
temperatures also led to higher oxygen consumption. There was an
efflux of ammonium and phosphate during all other incubations ex-
cept the Night incubation in the MHW chambers and the Day incu-
bations during the ‘Ambient MHW’. The effluxes were higher in the
heated chambers during the ‘Induced MHW ’. Mean silicate fluxes
were mostly positive across sampling occasions and day and night
incubations, except in the MHW treatment during both incubations
of the ‘Ambient MHW’.
Day/Night explained the highest proportion of 44.8% of vari-
ance for oxygen fluxes (Table 2). For the nutrient fluxes, Sampling
was the dominant factor explaining 55.7% of the variance, while
Treatment, although significant, only explained 1.2% of the vari-
ance. Significant interactions between Treatment × Sampling were
observed for oxygen and nutrient fluxes, suggesting that the
Treatment effects varied depending on the Sampling occasion.
However, the dispersion analysis indicated heterogeneity of vari-
ances for the majority of factors and interac tions, particularly re-
lated to Day/Night and Sampling, suggesting complex changes in
the variability of the responses. Notably, Treatment did not cause
a significant within- group dispersion for either oxygen or nutrients.
Together, the heterogeneit y of variance for Sampling and Day/
Night, reflecting the differing natural conditions causing complex
changes in the responses, and the significant but little explanatory
power of Treatment, due to four out of six samplings in the model
without active heating, warrants the need for a focused analysis of
the treatment effect on the subsets for Day/Night and Sampling in
the following section (Table 2).
In the Treatment- focused analyses, where each group of Day/
Night and Sampling was analysed separately, no significant differ-
ences between MHW and Control were detected on the first sam-
pling day (‘Acclimation’) in the multivariate analyses of the oxygen
and nutrient fluxes (NH4
+- N , P O 4
3−- P and Si) either during Day or
Night (Figure 5; Table 3). Oxygen was consumed both during the
night and during the day incubations, but considerably less in the
day incubation. Mean fluxes ( Table 1) of ammonium and phosphate
ranged between −0.2 and 0.1 (mmol m−2 day−1) during the night in-
cubation, and the ef flux of ammonium during the day was stronger.
The multivariate analyses of the combined nutrient fluxes
(NH4
+- N , P O 4
3−- P and Si) showed no significant difference between
the Control and the MHW during the ‘Induced MHW’ (Table 3), but
the effluxes in the MHW treatment were all higher. The oxygen
consumption in the MHW treatment was higher than in the Control
during the night- and daytime incubations during the ‘Induced MHW’
(Table 1). The multivariate analyses indicated significant differences
for the oxygen fluxes between Control and MHW for Day (pseu-
do- F 5 3.29, p 0.004) and Night (pseudo- F 15.78, p 0.014; Table 3).
When oxygen and nutrient fluxes during the ‘Induced MHW’ were
investigated together in the principal component analyses, a distinct
FIGURE 4 Density plots of the size distributions of the most
prevalent bioturbators (a) Hediste diversicolor, (b) Macoma
balthica and (c) Marenzelleria spp. for treatment (orange) and
control (blue). Macoma balthica and Marenzelleria spp. show higher
frequencies in smaller sizes in marine heatwaves (MHW) treatment
than in control. However, the PERMANOVA test only indicated
a significant difference for Marenzelleria spp. (R2 = 0.14, pseudo-
F = 12.81, p = 0.001).
0.51.0 1.52.0
2465
10 15
0.00 0.05 0.10
0.00.2 0.4
0.00.5 1.0
Size [mm] Size [mm]
density
Treatment
Control
MHW
(c)
Marenz. spp.
Size [mm]
(a)
Hediste d.
(b)
Macoma b.
TAB LE 1 Average and standard deviation of solute fluxes (mmol m−2 day−1 ) for each incubation.
Sampling Day/Night Treatment Oxygen Ammonium Phosphate Silicate
Acclimation Night Control −46.6 ± 5.9 0.0 ± 0.7 0.1 ± 0.1 2.2 ± 2.3
MHW −43.5 ± 11.1 −0.2 ± 1 .1 −0 .1 ± 0.6 0.2 ± 2.3
Day Control −7. 4 ± 11.8 0.8 ± 0.5 0.1 ± 0.1 1.5 ± 1.9
MHW −3.7 ± 9.1 0.7 ± 0.5 0.1 ± 0.1 0.5 ± 2.9
Induced MHW Night Control −46. 2 ± 7.0 5.3 ± 5.2 0.7 ± 0.7 7. 5 ± 5.3
MHW −92.2 ± 12.2 9. 9 ± 3.2 0.9 ± 0.4 13.1 ± 6.0
Day Control −10. 9 ± 14.1 4.8 ± 3.2 0.4 ± 0.3 14. 0 ± 5.0
MHW −4 7. 0 ± 14. 6 8.4 ± 2.5 0.7 ± 0.3 16 .5 ± 3.2
Ambient MHW Night Control −92.0 ± 15.0 2. 2 ± 0.9 0.0 ± 0.2 4.6 ± 5.5
MHW −94.1 ± 23.3 1.9 ± 1.5 0.1 ± 0.2 −1 . 0 ± 2.4
Day Control 119.7 ± 31 .2 −2 .0 ± 2.1 −0.3 ± 0.5 1.7 ± 4.3
MHW 144 .8 ± 42.1 −1. 1 ± 1.1 −0. 2 ± 0.2 − 0.1 ± 5.2
Note: Colour gradient indicates marine heatwaves (MHW) c ategor y (yellow = moderate, orange = strong, dark red = severe) during the sampling.
|
7
GÖBELER et al .
response was shown, mostly driven by changes in ox ygen fluxes,
with the first two principal components capturing a large proportion
of the variance (Figure 6, PC1: 66%, PC2: 23.4%).
In the last sampling 3—‘Ambient MHW’, all chambers had for
6 days followed the ambient temperature, which peaked at 24°C on
the sampling day, corresponding to a strong MHW (and 6°C higher
than in sampling 1). In addition, a strong phytoplankton bloom had
developed. Overall, there were no significant differences between
the treatments with regard to the ox ygen and nutrient fluxes within
the respective time of incubation (Day/Night; Table 3). Most in-
dicative of the strong phytoplankton bloom was the high oxygen
production in both Control and MHW chambers during the day in-
cubation. Furthermore, comparably strong influxes were observed
in both MHW and Control for ammonium and phosphate.
4 | DISCUSSION
This study is the first to investigate the effects of MHWs on natu-
ral benthic communities and their ecosystem functioning in terms
of nutrient cycling and metabolism in situ by using a unique sys-
tem consisting of large (50 cm), custom- made benthic chambers
combined with a domestic underfloor heating system (Göbeler
et al., 2023). While the MHW did not cause any major shift in the
communit y structure, despite subtle signs of mortality in late life-
stage Marenzelleria spp. based on a change in the size distribution,
significant amplifications in metabolic rates and nutrient fluxes
were observed. Specifically, the results indicate that heterotrophic
processes were significantly influenced by the MHW, whereas au-
totrophic processes remained unaffected. These findings highlight
that func tional shift s in ecosystem processes are likely to precede
structural changes in the community.
While temperature is a key determinant of the structure, func-
tion and interactions in marine ecosystems, it is but one of many
stressors currently threatening the oceans. Disentangling the indi-
vidual impact of temperature, particularly when considering highly
complex marine ecosystems, remains challenging. Moreover, the
complexity of the impact is even more pronounced when it does not
cause mass mortalities that have been observed after extreme sum-
mer MHWs (Cavole et al., 2016; Garrabou et al., 2009, 2019; Genin
et al., 2020; Rogers- Bennett & Catton, 2019; Seuront et al., 2019).
Impacts can be ecologically meaningful, but more subtle, and are
then more challenging to detect. Laboratory studies contribute to
the understanding of the mechanistic effects of temperature but in-
troduce artefacts and disturbances, particularly when working with
benthic communities, for example when sediment cores are taken
from the field to the laboratory. Specifically, the retrieval of sediment
cores from the field can cause disturbances to the delicate benthic
boundary layer and sediment matrix, for example disrupt burrows
built by macrofauna, change porewater dynamics and cause unnat-
ural resuspension of surface sediment (Blomqvist, 1991). Due to the
highly heterogeneous and patchy nature of the seafloor, achieving
representative results often requires a large number of replicates.
This challenge is compounded by the typically small diameter of
tube corers. Further, the smaller the corer, the higher the friction
impact of the corer wall, causing a compactization of the sediment
(Blomqvist, 1985). While larger box corers could improve represen-
tativeness, they are less frequently used due to the heavy infrastruc-
ture required (but see Kauppi et al., 2023). By employing an in situ
approach with relatively large chambers, this system minimizes the
artefacts introduced through laboratory techniques and allows for
capturing the natural complexity and heterogeneity of benthic eco-
systems. While the in situ system also introduces artefacts, such as
somewhat reduced water circulation, the system nevertheless pro-
vides a more realistic depiction of ecosystem effects of MHWs than
laboratory experiments (Göbeler et al., 2023).
The response of benthic invertebrate communities to MHWs can
vary depending on the duration and intensity of such events and
can range from anything between mass mortalities to habitat range
FIGURE 5 Solute fluxes[mmol/m2 day−1] (in order from upper to
lower panel: Oxygen, ammonium, phosphate, silicate) during night-
time (left column, grey back ground) and daytime (right column,
yellow back ground) incubations of each sampling day. On day 1.
‘Acclimation’, when all chambers (n = 10) were at same conditions at
about 18°C. After 6 days of heating the marine heat waves (MHW)
chambers (n = 5) to about 26°C ‘Induced MHW’ (2.), while control
chambers (n = 5) were at about 21°C. After another 6 days, when all
chambers were back to natural conditions at about 23°C during an
‘Ambient MHW’ (3.) together with a strong phytoplank ton bloom.
-100
0
100
200
-5
0
5
10
-1
0
1
-5
0
5
10
15
20
Sampling
[
mmol
m
−
2
d
−
1
]
Treatment Control
MHW
Night
Day
Oxygen
Ammonium
Phosphate
Silicate
1.
2.
3.
1.
2.
3.
xulF
8
|
GÖBELER et al .
expansion (reviewed in Smith et al., 2023). In our case, no severe shift
in the macrofaunal community compared with the Control, based
on abundance or biomass, was observed when exposing this cold
temperate benthic communit y to 6 days of a severe MHW of about
26°C. Furthermore, we did not observe any mor tality (e.g. surfaced
animals) over the course of the experiment or during the final sam-
pling of the sediment. Organisms from highly variable environment s
can have a higher tolerance to extreme conditions (Stuart- Smith
et al., 2017). Possibly, the increased frequency of extreme tempera-
tures particularly over the last two decades in this area (Goebeler
et al., 2022) has contributed to adaptations in the benthic community.
These adaptations can occur through selection over several gener-
ations (Coleman & Wernberg, 2020), cross- generational plasticity
from parent to of fspring or as carry- over effects within the devel-
opment of the larvae (Byrne et al., 2020), sometimes also referred to
as an ecological memory (Jackson et al., 2021). However, we noticed
a significant shift in the size distribution of Marenzelleria spp. in the
MHW treatment towards a higher frequency of smaller individuals.
Marenzelleria spp. larvae typically settle in the period of March–May
and have a maximum observed lifespan of 403 days af ter settling at
a nearby sandy site (Kauppi et al., 2018). This suggests a presence of
individuals both at the early and at the late stages of their lifetime in
Parameter Sampling Day/Night R2Pseudo- F p(perm) p(disp)
Oxygen Acclimation Night 0.04 0.32 0.657 0.583
Day 0.04 0.31 0.529 0.694
Induced MHW Night 0.87 53.29 0.0 04 0.517
Day 0.66 15.78 0.014 0.897
Ambient MHW Night 0.00 0.03 0.871 0.822
Day 0.13 1.15 0.343 0.632
Nutrients Acclimation Night 0.10 0.92 0.523 0.616
Day 0.03 0.27 0.720 0.310
Induced MHW Night 0.17 1.61 0.235 0.310
Day 0.25 2.73 0.080 0.293
Ambient MHW Night 0.24 2.56 0.120 0.507
Day 0.04 0.31 0 .76 4 0.464
Note: R2 indicates the proportion of variance explained, pseudo- F shows the test statistic,
permutation derived p- value p(perm) the signific ance and p(disp) the dispersion test. Colour
gradient indicates MHW category (yellow = moderate, orange = s trong, dark red = severe) during
the sampling. Signific ant when p < 0.05 are marked in bold.
TABLE 3 PERMANOVA results of
the effect of treatment on ox ygen and
combined nutrient fluxes by sampling and
the time of incubation (Day and Night).
Parameter Tes t R2Pseudo- F p(perm) p(disp)
Oxygen Treatment 0.003 3.030 0.001 0.534
Day/Night 0.448 412.672 0.001 0.006
Sampling 0.140 64.591 0.001 0.001
Treatment × Day/Night 0.002 1.578 0.246 0.024
Treatment × Sampling 0.023 10.663 0.002 0.001
Day/Night × Sampling 0.331 152.366 0.001 0.001
Treatment × Day/
Night × Sampling
0.001 0.611 0.605 0.12
Nutrients Treatment 0.012 1.723 0.001 0.215
Day/Night 0.016 2.400 0 .113 0.788
Sampling 0.557 41 .428 0.001 0.001
Treatment × Day/Night 0.002 0.288 0.685 0.674
Treatment × Sampling 0.044 3.24 0 0.049 0.021
Day/Night × Sampling 0.041 3.081 0.053 0.003
Treatment × Day/
Night × Sampling
0.005 0. 361 0.779 0.16 4
Note: R2 indicates the proportion of variance explained, pseudo- F shows the test statistic,
permutation- derived p- value p(perm) the significance and p(disp) the corresponding analyses of
dispersion. Significant when p < 0.05 are marked in bold.
TAB LE 2 Three- way PERMANOVA
results testing the effect of Treatment ,
Day/Night, Sampling and their
interactions, constrained by chamber, on
oxygen and combined nutrient fluxes.
|
9
GÖBELER et al .
our study. While speculative, the lack of large individuals in the MHW
treatment could be a sign of mortality, although more replicate core
samples would have been necessary to confirm that. They could also
have burrowed deeper into the sediment than the cores sampled to
escape the heat stress (based on our personal observations on how
deep they burrow in this area), although this cannot be confirmed.
Either way, if these individuals died or had to invest a lot of energy
to escape before they had the chance to reproduce (Sokolova, 2013),
this could have important implications for the whole future popu-
lation of Marenzelleria spp. The individuals collected in this study
likely belong to Marenzelleria viridis or Marenzelleria neglecta (Kauppi
et al., 2018), which can build deep burrows to a maximum of 20 cm
or 15 cm, respec tively (Renz & Forster, 2013). Thus, the loss of the
large individuals also has impor tant implications for the bioirrigation
capacity of the sediment (Bernard et al., 2019). Yet, as no treatment
effect was observed for the whole macrofaunal community, except a
change to smaller individuals of Marenzelleria spp. in the MHW tre at-
ment, we can still assume that generally similar communities were
present throughout the experiment when interpreting the oxygen
and nutrient dynamics. Nevertheless, shifts in size distribution can
have particularly large functional consequences that warrant atten-
tion in this type of study (Norkko et al., 2013). The three dominant
species (Macoma balthica, Hediste diversicolor and Marenzelleria spp.)
naturally have different modes and rates of bioturbation, but in this
study, their relative contributions to the measured fluxes c annot be
distinguished. The potentially species- specific sensitivity of biotur-
bation rates to MHW impacts will have to be addressed in future
studies.
The direction and magnitude of changes in the oxygen dynamics
are a strong prox y for the activity and metabolism of macro- and
meiofauna (Rodil et al., 2020), as well as microbes (Seidel et al., 2023),
and are tightly linked to temperature as well as primary production
during daytime. In this study, we investigated the oxygen dynamics
of bare- sediment, benthic communities over 2 weeks encompassing
three different increasingly warmer temperature regimes across
different light regimes. The overall model revealed that light condi-
tions were the dominant factor explaining the variance, as naturally
during day- and night- time different metabolic processes dominate.
During night- time, photosynthesis is inactive and respiration is the
dominant metabolic process. Consequently, negative oxygen fluxes
were observed in all night- time incubations. Community respiration
was markedly influenced by temperature and almost twice as high in
the MHW treatment during the induced MHW over a temperature
difference of 4.3°C compared with the Control. The PCA analysis of
the oxygen and nutrient fluxes during the induced MHW showed
that temperature and Day/Night greatly dominate the separation of
variables along the two axes, together accounting for 89.4% of the
variance. The response of the benthic community and the magnitude
thereof is in line with other in situ heating experiments even when
conducted in different habitats, for example a vegetated commu-
nity in a shallow, macrotidal and sheltered bay in southern Spain or
a tropical reef flat close to Heron Island, Australia (Egea et al., 2023;
Trnovsky et al., 2016). The daytime oxygen fluxes were compara-
bly less negative than during the night- time incubations, as light is
available and oxygen is produced through the photosynthetic ac-
tivity of the microphytobenthos and phytoplankton. Consequently,
the observed rates represent the net primary production. The ab-
solute dif ference between average ox ygen consumption of day-
and night- time incubations, that is oxygen production, during the
‘Induced MHW ’ was similar for both Control and MHW treatments
FIGURE 6 Principal component
analyses of normalized solute flux rates
of day- and night- time incubations during
the ‘Induced MHW’, where the marine
heatwaves (MHW) treatment (orange)
experienced temperature about 5°C
higher than control (blue). The vectors
in the plot represent the directions of
maximum variance for each of the original
variables (O2, NH4
+- N, PO4
3- P − and Si)
in the space defined by the first two
principal components.
Day
Night
Day
Night
Si flux
PO flux
NH flux
O flux
2
4
3-
4
+
Night
Night
Night
Night
Night
Night
Night
Night
Day
Day
Day
Day
Day
Day
Day
Day
-2
-1
0
1
2
-3 -2 -1
012
PC1: 66 % variance
PC2: 23.35 % variance
Treatment
aa
Control
MHW
10
|
GÖBELER et al .
with 35.3 ± 10.8 and 45.3 ± 12.0, respectively. This is under the
assumption that the respiration rates remained constant, but light
respiration rates can be higher than during the night (Fenchel &
Glud, 2000). No notable increase of oxygen production in the MHW
chambers compared with Control agrees well with the general con-
cept that heterotrophic processes increase more strongly with ris-
ing temperatures compared with autotrophic processes (Alsterberg
et al., 2012; Hancke & Glud, 2004; Prelle et al., 2019; Trnovsky
et al., 2016; Yvon- Durocher et al., 2010).
Ecosystem func tio ning of benthic communities in te rms of nutri-
ent cycling is highly complex and determined by various biotic and
abiotic factors (Gammal et al., 2019). In this study, relatively natural
oxygen conditions inside the chambers were maintained through the
water exchange with the surrounding water for the majority of the
time (unless incubating) and followed the ambient conditions (see
Göbeler et al., 2023 for details). The three- way PERMANOVA re-
vealed a significant effect of the Sampling occasion and Treatment
on ecosystem functioning, while Sampling accounted for the ma-
jority of the variance. Treatment was not significant when analysed
within each Sampling. This could suggest that Treatment had a
weak, consistent general effect on ecosystem functioning but was
masked by the natural variability within the relatively small subsets
and the complexity of the benthic community response. The solute
fluxes behaved similarly across day- and night- time and will not be
separately distinguished in this discussion unless further specified.
The mean solute fluxes of silicate, phosphate and ammonium were
positive, thus an efflux, during the first incubation (all chambers at
about 18°C) resembling known biogeochemical dynamics in coastal
areas during the summer (Høgslund et al., 2023; Kauppi et al., 2017;
Rios- Yunes et al., 2023; Tallberg et al., 2017). The mean and th e st an-
dard deviation of the effluxes (Table 1) increased in both Control
and Treatment during the induced MHW compared with Acclimation
fluxes, respectively. This increase was stronger for MHW cham-
bers, yet insignificant compared with Control. This suggest s a
temperature- dependent increased metabolization of organic mate-
rial, even though this experimental site is low in organic material,
through macrofaunal and microbial activity (Seidel et al., 2023). This
is reflected in the aforementioned increased community respiration.
Kauppi and Villnäs (2022) found a reduced metabolic activity in a
laboratory study on intact cores of a benthic community from a 32 m
deep muddy site, exposed to an experimental MHW at about 17°C
compared with the control, which was at the normal temperature of
about 8°C. Thus, an MHW with 26°C at 2.5 m depth boosted the re-
lease of nutrients from the sediment, whereas a temperature of 17°C
at a 32 m deep site caused a reduction in the mineralization activity.
This highlights the importance of understanding the environmental
history, context and conditions on a small spatial scale, particularly in
coastal areas (Gammal et al., 2019). Fur thermore, in the case of tem-
perature this highlights the role of acknowledging that the climato-
logical values vary bet ween sites and that MHWs will have dif fering
effects on marine ecosystems and their functioning.
Following the ‘Induced MHW’, the heating was terminated and
all chambers followed ambient water temperatures for 5 days until
th e fina l inc uba t io n s. Du r ing this time, the am b ien t te mperat ure con-
tinued to increase to 24.2°C and a strong phytoplankton bloom was
observed towards the end of the experiment. Nutrient and oxygen
fluxes were not different between the MHW and Control treatments
during this sampling (Table 3). Our ability to distinguish the effect of
the phy toplankton bloom from the relatively high temperatures is
limited to speculation as key variables were not measured (e.g. due
to failure halfway through the experiment of instrument measuring
ambient chlorophyll a). The presence of the phytoplankton bloom
likely changed the direction and magnitude of the solute fluxes and
oxygen rates compared with previous sampling occasions. The com-
bination of high oxygen production rates and mostly negative nutri-
ent fluxes during the daytime could suggest a strong activity, growth
and therefore nutrient uptake through the phytoplankton. Warming
increases metabolic rates of plankton and can, in nutrient- rich con-
ditions, increase phytoplankton biomass (Hayashida et al., 2020;
Lewandowska et al., 2014). This is further supported by the highest
measured respiration rates in this experiment during the night incu-
bation, possibly caused by the elevated amount of organic matter
to be degraded due to higher primary production, even when the
temperature was about 3°C lower than during the ‘Induced MHW’
treatment. Assuming that the nutrient fluxes behaved comparably
to the strong effluxes at higher temperatures during the ‘Induced
MHW’ without the presence of a phytoplankton bloom, the uptake
rates by the phytoplankton might be even more pronounced.
5 | CONCLUSIONS
Pr evi ous st udi e s exa min i ng th e impa c t of MH Ws on eco s yst e m fun c-
tioning in benthic ecosystems have mostly relied on laboratory tech-
niques and resulted in more or less distinct responses of increased
communit y respiration and nutrient effluxes (Dolbeth et al., 2021;
Kauppi et al., 2023; Kauppi & Villnäs, 2022). Our study utilized a
unique system, combining large custom- made benthic chambers
with domestic underfloor heating, to investigate the effect of MHWs
on natural benthic community struc ture and ecosystem functioning
(nutrient cycling) in situ under realistic conditions. Furthermore, it
enabled us to measure these responses under natural light and dark
conditions and encompassing varying ambient conditions with re-
gard to temperature and a strong phytoplankton bloom. Contrary
to our expectations, when exposing a natural benthic community
to temperatures about 5°C higher than the Control treatment (and
about 10°C above the climatological mean temperature), we did not
observe any substantial changes in the community structure nor a
breakdown of metabolic processes. While we did not observe any
major treatment effect in the community structure, we did observe
a change in size distribution of Marenzelleria spp. towards a higher
frequency of smaller individuals (or lower frequency of large indi-
viduals), which might indicate a greater effect on the population
dynamics of Marenzelleria spp. However, the effects of MHWs on
life history traits of the different species warrant more research.
Our results highlight that coastal areas with sandy sediments, low
|
11
GÖBELER et al .
in organic mat ter content, respond to elevated temperatures with
increased communit y respiration and nutrient release. At the same
time, primary produc tion appeared to remain stable even under el-
evated temperatures during this shor t- term MHW, suggesting that
heterotrophic processes are boosted more by increasing tempera-
ture than autotrophic processes. Considering that these effects were
observed in response to an MHW with a minimum duration and that
the experiment was conducted in a relatively homogeneous benthic
habitat, future studies need to broaden the spectrum of measured
parameters and other habitats to gain a more holistic understanding
of the impacts of longer- lasting MHWs on coastal marine ecosys-
tems. Importantly, even though our experiment was relatively short
term, we were able to observe some significant sublethal effects.
In a longer experiment or longer real MHW, multiple subtle effects
would have time to manifest, which may have long- term impacts that
contribute to eroding ecosystem functionality.
AUTHOR CONTRIBUTIONS
Norman Göbeler was responsible for the analysis of the data and
led the writing of the manuscript. Data acquisition was done by Alf
Norkko, Laura Kauppi and Norman Göbeler. Joanna Norkko acquired
fu ndi ng fo r the proj ect. Al l aut hor s con tri but ed to the co nce ptu ali za-
tion of this study, interpretation of the result s, critically reviewed
and edited the draf ts, and accepted the final ver sion for publication.
ACKNOWLEDGEMENTS
We greatly appreciate the support from Onni Talas Foundation in-
terns Emma Forss, Anni Leinonen, Neea Hanström and Katariina
Myyr y under the supervision of Hanna Halonen during the sam-
pling. We also thank Judi Hewitt (University of Auckland) for the
discussions and advice on statistics. This study was supported by
the Walter and Andrée de Nottbeck Foundation and the European
Union's Horizon 2020 research and innovation programme under
grant agreement No.: 730984, the ASSEMBLE Plus project . This
study utilized research infrastructure facilities at Tvärminne
Zoological Station, University of Helsinki, as par t of FINMARI
(Finnish Marine Research Infrastructure consortium). This is a pub-
lication from the Centre for Coastal Ecosystem and Climate Change
Research (w w w . c o a s t c l i m . o r g ).
CONFLICT OF INTEREST STATEMENT
All authors declare no conflicts of interest .
DATA AVA ILAB ILITY STATE MEN T
The dat a and R- code supporting the findings in this study are avail-
able from the Dryad Digit al Repositor y: h t t p s : / / d o i . o r g / 1 0 . 5 0 6 1 /
dryad. kd51c 5bj0 (Göbeler et al., 2025).
STATEMENT OF INCLUSION
All aut ho rs are from and/or live in Fi nland, where th e stud y wa s con-
ducted. Furthermore, all analyses and related work were conducted
in Finland.
ORCID
Norman Göbeler https://orcid.org/0000-0001-9164-5370
Alf Norkko https://orcid.org/0000-0002-9741-4458
Joanna Norkko https://orcid.org/0000-0001-9885-8408
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SUPPORTING INFORMATION
Additional supporting information can be found online in the
Suppor ting Information section at the end of this article.
Table S1. Daily average temperature and standard deviation per
treatment (n = 5) of benthic chambers and climatological values
based on the reference period 1931–2020 (Göbeler et al., 2022).
Table S2. Loading values of oxygen and nutrient fluxes on the first
four principal components derived from the PCA conducted on the
“induced MHW” Sampling in Figure 6.
How to cite this article: Göbeler, N., Kauppi, L., Norkko, A., &
Norkko, J. (2025). Marine heatwaves amplif y benthic
communit y metabolism and solute flux in a seafloor heating
experiment. Functional Ecology, 00, 1–13. h t t ps : //d o i .
org /10.1111/13 65-2435 .70 028
