Climate change and fisheries affect benthic composition and diversity in the North Sea - Investigations at the Dogger Bank during three decades (1991-2021)

doi:10.21203/rs.3.rs-3086664/v1

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Climate change and fisheries affect benthic composition and diversity in the North Sea - Investigations at the Dogger Bank during three decades (1991-2021)

https://doi.org/10.21203/rs.3.rs-3086664/v1

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Over a period of 30 years (1991–2021) benthic fauna was monitored at a long-term sampling station at the NATURA2000 marine protected area Dogger Bank and at a reference station in Dutch waters, using a ring dredge and a 2 m beam trawl. Changes of benthic diversity of 127 species were detected in 238 temporally distributed samples and correlated with temperature and intensity of bottom fishing. Our results provide a solid benchmark for future research in the German Exclusive Economic Zone. During the timeframe of our study, the temperature at depths below 30 m remarkably increased by 4°C, while fishing pressure in the studied ICES rectangles decreased over the past 30 years from over 100.000 h of trawling per year to about 20.000 h. The numbers of benthic individuals sampled each year show a negative trend, while the number from 1991 to 2001 is first more influenced by fishing, later by temperature. An analysis of the benthic fauna at some stations adjacent to the long-term sampled station, sampled in 2021, showed that species composition is comparatively uniform over space and that therefore long-term sampling of one station can be used as a proxy for its surroundings in this area.

Dogger Bank

North Sea

benthic epifauna

long-term analysis

climate change

fishing pressure

The North Sea is a shallow, semi-enclosed shelf sea (Wieking & Kröncke 2005; Ducrotoy et al. 2000). The Dogger Bank (Fig. 1), is an isolated (Cotterill et al. 2017) 300 km wide sandbank (Kröncke & Knust 1995) at the central-southern North Sea. It is a Pleistocene-aged moraine (Stride 1959) with Holocene and modern sand deposits (Fitch et al. 2005). According to Sonnewald & Türkay (2012), the Dogger Bank is considered a valid area to conduct long-term studies, as a uniform, sandy and shallow sediment structure is predominant. Anthropogenically influenced marine ecosystems witness a loss of general biodiversity (Coleman & Williams 2002; Worm et al. 2006). The North Sea is considered to be strongly anthropogenically influenced by industry, pollution from agriculture (Ducrotoy et al. 2000), marine traffic (Gislason 1994), offshore wind farms (Krone et al. 2013) and pipelines (Lacey & Hayes 2020). One of the major impacts on benthic fauna are bottom fisheries (Tillin et al. 2006). In order to scientifically accompany the planned exclusion of bottom-contacting fisheries in the NATURA2000 marine protected areas (MPA’s) of Germany’s exclusive economic zones (EEZ’s), various studies are conducted in under the umbrella of the DAM (Deutsche Allianz für Meeresforschung) pilot project "MGF-North Sea". However, this exclusion is not yet in place (Sander et al. 2022) in contrast to the British area of the Dogger Bank, where the exclusion of bottom fisheries is in force since mid-2022 (Marine Management Organization 2022). The increase of OSPAR-protected (Oslo-Paris commission for the protection of the marine environment of the North-East Atlantic) marine areas decided in 2010 was not achieved either, so there are still not protected gaps of nature reserves despite the already protected areas (OSPAR ministerial conference 2010). In this study, a historical comparison of the benthos at the eastern part of the Dogger Bank, the so-called "tail end" (Kröncke 1990), was made. Studies regarding the change of macrofauna communities were already conducted for the 21th century (Kröncke 2011). While it was stated that in the 21st century the benthic fauna in the North Sea was monitored still irregularly (Callaway et al. 2002), other studies state that the North Sea is one of the best studied seas in terms of fisheries (Nielsen et al. 1993). However, regular recording of data is important to visualize temporal changes and to preserve marine ecosystems (Reiss et al. 2009). Therefore, 37 stations at the Dogger Bank in British, Dutch and German waters have been regularly (annually, with some interruptions) visited and benthic fauna was sampled since 1991. At the easternmost station (listed below as station 40) of that Dogger Bank long-term monitoring project, investigations per cruise were intensified during some years. Here, sampling occurred every three hours for a time frame of 48 hours, from 1994 to 2012. This station is located inside the German NATURA2000 marine protected area (MPA). In the present study, the data on benthic diversity of this station, as well as at a 30 nm distant reference station at Dutch waters (same depth and sediment structure) at the Dogger Bank tail end, are evaluated and related to changes in fishing pressure and temperature data covering 20 sampling campaigns from 1991 to 2021.

This baseline study will allow comparison of changes in the benthic fauna after the exclusion of mobile, bottom-contacting fisheries. According to available data of fishing effort in the surrounding area, trawling intensity declined sharply since 2001 (Couce et al. 2019; Fig. 2). Hence, this study over a longer time period reveals first predictions regarding the consequences of a reduction of trawling activity frequency. All species over one centimeter in size that live in the uppermost sediment layer (first 5 cm) of the seafloor or on the seafloor including demersal fishes that could be captured were examined. These species in the following are summarized as benthos or benthic fauna, respectively. The benthos was expected to differ little between the two stations due to the same habitat characteristics. Nevertheless, our assumption that the benthos changed over the last 30 years with changing environmental parameters and fishing pressure over time is supported by the results herein.

During three decades from 1991 to 2021 the research vessels "Heincke" and "Senckenberg" have visited the Dogger Bank (Fig. 1) 29 times, whereas station 40 was sampled with two different nets during 20 years, almost every year. Additionally, beam trawl sampling was repeated at the same track of station 40 every three hours for up to 17 times per cruise in the years 1994 to 1997, 1999, 2001 to 2004, 2006, 2008 to 2010 (winter and summer), 2011 and 2012 (Table 1). Station 36 (our reference station) was sampled 18 times in summer (July / August) and two times in winter (February) between 1991 and 2022. In the years in which Station 40 was sampled several times in succession, all data were nevertheless used, but averaged for the evaluation. For the evaluation, only the expeditions carried out in the summer months were used, as the temperature was also only analysed in the summer months, thus ensuring a better interpretation of the results. The exact overview about the number, times and locations of sampling can be found in Table 1.

Sampling was performed with two different gears. First, a ring dredge (diameter 1 m, 1 cm² (0.8 cm² in the cod end) mesh size) was used to collect the main taxa of the epi- and infauna. For this purpose, the dredge was lowered to the seafloor and then pulled slowly (~ 1.5 kn) in one direction for 3–5 minutes. The dredge penetrates up to about 50 cm into the sediment of the seafloor. Samples were sorted and species determined on board as quickly as possible and then the fauna was released back into its natural habitat when possible. During this process, the species found were recorded in a presence/absence matrix. Species that could not be determined immediately on board or were of special scientific interest were fixed in 96% EtOH or in a ~ 5% Formaldehyde sea water solution and then transferred to the laboratory of the Senckenberg Research Institute (SRI) in Frankfurt am Main for later identification. In addition, the benthos was collected with a 2 m beam trawl (cod end mesh size 1 cm²), being towed over the seafloor over a distance of 1 nm in one direction at 2 knots of speed. The catch was photo-documented and its volume determined. Afterwards the catch was sieved over two different mesh sizes (1 cm², 1 mm²), while a part of the fine fraction (> 1 mm) was immediately preserved. Species of the larger fraction (> 1 cm) were identified to species level as quickly as possible and then released to sea if not of further interest. All non-colonial forms were recorded by their abundance. Undetermined species, species of scientific interest and the fine fractions were fixed in a Formaldehyde- sea water solution and in recent years predominantly in 96% EtOH for subsequent determination and analyses. After identification, preserved specimens of the ring dredge and beam trawl were added to the diversity dataset already obtained on board.

The temperatures measured at each station with a CTD-probe were evaluated in terms of their changes per water depth and then averaged to determine long-term changes over the period studied (Figs. 3 and 4). In order to obtain the most accurate data possible regarding trawling activity in the area studied, the data from Couce et al (2019) were used. They provide reconstructed datasets showing the bottom contact fishery with beam and otter trawls in hours per year per ICES rectangle. The corresponding ICES rectangles around the sampled stations were determined and the corresponding data from 1991–2015 (end of dataset) were used.

Table 1

Number of samples and sampling years of station 40 and station 36 at the Dogger Bank
Year	Samplings station 40	Samplings station 36	Year	Samplings station 40	Samplings station 36
1991	1	1	2004	13	1
1992	1	1	2006	17	1
1994	9	1	2008	4	1
1995	19	1	2009	16	1
1996	17	1	2010	19	2
1997	16	1	2011	9	1
1999	16	1	2012	16	1
2001	16	1	2017	1	1
2002	9	1	2020	1	1
2003	17	1	2021	1	0

Statistical analyses

All statistical analyses were performed with R (v. 4.2.0, 2022). Therefore, the packages psych (v. 2.2.5), stats (4.2.0) and vegan (v. 2.6-2) were used. To test for differences in means, a Levene test was performed first to ensure homogeneity of the data. Then, a Two Sample t-test was performed to detect any significant differences or similarities in the two stations studied. For the statistical analysis of correlations, the correlation coefficient was determined. The temperature data of the two studied stations were first examined with an ANOVA (analysis of variance), then a correlation over the years was examined with the determination of the Spearman correlation coefficient. For the similarity analysis methods according to Bray & Curtis (1957) were applied.

Ethical statement

The care and use of experimental animals complied with German animal welfare laws, guidelines and policies as approved by § 7a Section 1 No. 1–8 Animal Welfare Act. Care was taken to ensure that no animal suffered unnecessarily and that all animals were released back into their natural habitat after a quick identification, as long as they could be identified properly.

Trawling activity decreases sharply at both stations studied from 1991 to 2015 (fig. 2). Whereas at the beginning of the sampling period up to almost 150,000 hours per year were fished in the corresponding ICES rectangles (fig. 1), in recent years only about 20,000 hours per year are fished in the same areas with beam and otter trawls.

For the correlation of temperature with the number of benthic species, a stratified analysis of water temperatures in the summer months (June 01 – August 31) in all years at a section of the study area was performed with Ocean Data View 5.5.2 (fig. 3). In summer, the surface temperature is different from the one in deeper layers. At about 25 m, a thermocline was detected and for the following analyses, the temperature data from 25 m and deeper were used.

An evaluation of the mean water temperatures from 1991-2021 for the summer months of June-August for stations 40 (ICES transects 40F3, 40F4, 39F3, and 39F4) and station 36 (ICES transects 40F2, 40F3, 39F2 and 39F3), below 25 m depth, shows that the mean temperature was increasing about 4 °C with time (fig. 4).

3.1 Species composition

A total of 115 species were recorded during the years 1991-2021 from station 40, consisting of the phyla Chordata (31 species), Arthropoda (26 species), Mollusca (21 species), Cnidaria (11 species), Echinodermata (10 species), Annelida (8 species), Bryozoa (6 species) and Porifera (2 species). From station 36, a total of 72 species were identified during the sampling period 1991-2020. These belonged to the phyla Chordata (24 species), Mollusca (14 species), Arthropoda (12 species), Echinodermata (9 species), Cnidaria (4 species), Bryozoa (3 species), Annelida (2 species), Nemertea (1 species) and Porifera (1 species). For both station 36 and 40, the most common (in every year present) species were Arnoglossus laterna (Walbaum, 1792), Asterias rubens Linnaeus, 1758, Buglossidium luteum Chabanaud, 1930, Limanda limanda Linnaeus, 1758 and Pagurus bernhardus (Linnaeus, 1758) (fig. 5).

The species discovery (or so-called power-) curves of the two investigated stations showed rather similar results (fig. 6). While no new species have been recorded at station 40 since 2017, the species discovery curve at station 36 is not yet fully asymptotic, as additional species have also been identified in recent years. This difference comes mainly due to the high number of repetitive samples at station 40, as explained in the methods section. In spite of their difference in the number of (detected + new) species, both curves are showing a rather similar shape.

The number of individuals divided by the number of samples is higher at station 36 than at station 40 (fig. 7). However, the decreasing trend at station 36 is also steeper than at station 40.

3.2 Biodiversity

An analysis of the Shannon-Wiener biodiversity indices over the period 1991-2021 showed that the biodiversity index for both stations generally shows a slightly negative trend (fig. 8). However, in 2021 a strong increase of the biodiversity index can be seen again which was even slightly above the starting value from the year 1991 at station 36. The means of the two stations are not significantly different (p= 0.3837), indicating no difference in mean Shannon-Wiener biodiversity.

Correlation of Shannon-Wiener indices with trawling activity in the studied areas showed no statistically significant relationship. At both stations the determination of Pearson's correlation coefficient was clearly above the 0.05 mark (station 36: p=0.4068, station 40: p=0.1679).

A Bray-Curtis biodiversity analysis of the two stations over the whole sampling period shows that biodiversity changes over time at both stations, while both stations also differ from each other (fig. 9). The stress value with 0.08 is relatively low.

When correlating certain species with trawling intensity at station 40 it is noticeable that some species show a positive correlation with increasing fishing pressure, others relate negatively. The dominant, small flatfish species Buglossidium luteum and Arnoglossus laterna are among the species that show a negative correlation with increasing trawling activity. The opposite is observed for Asterias rubens and Pagurus bernhardus.

A comparison of the mean values with an ANOVA shows with a p-value of < 0.05 that the mean values of the temperatures of the two stations differ. A determination of the Spearman correlation coefficient clearly shows that the temperature increases within the years 1991-2021 (station 36: p=0.0001138, station 40: p=0.00379).

Dogger Bank sampling 2021

In 2021, the benthic fauna of Dogger Bank was surveyed at 25 stations (fig. 10). The stations were chosen within and close to the focus area of the MGF-North Sea project. Furthermore, the stations are located around the historically long-term sampled station 40. This was to ensure that a comparison of a station over time can be counted representatively for the Dogger Bank as it is assumed that the benthic fauna is comparatively uniform. Because of organisatoric difficulties, two different mesh sizes were used in the beam trawl in the stert. 14 stations were sampled with a mesh size of 0.8 cm, 11 stations with a mesh size of 1 cm as in the last decades (fig. 10).

A Bray-Curtis analysis showed that the benthic biodiversity of the sampled stations is close together except for two stations (fig. 11). The conspicuous stations 1 and 13 are both located at the outermost corners of the investigated area on Dogger Bank (fig. 10). The two clusters for the remaining stations come because two different mesh sizes at the cod end (0,8 mm at stations marked in red and 1,0 cm at stations marked in blue) were used in order to compare the beam trawl catching efficiency. Thus, the abundances varied because of the used methodology. The species observed overlapped 47 % at all stations, regardless of the different mesh sizes. However, it is noticeable that a mesh size of 0.8 cm ensures a higher species diversity in the catch, 38 % of all recorded species are only found in the catches with the mesh size of 0.8 cm, while only 15 % of the species were found in the usual 1 cm mesh sizes.

A comparison of the Shannon-Wiener indices (Hs) of the stations also expresses that station 1 as well as station 13 shows a slight difference in biodiversity compared to the stations with a lower Hs. In general, there are only very slight differences in the Hs of the stations studied (fig. 12).

During the sampling of station 40 and reference station 36, some species (Arnoglossus laterna, Asterias rubens, Buglossidium luteum, Limanda limanda and Pagurus bernhardus) were present in each sampling year. These species were the same at both stations, suggesting that the most abundant species are also widely distributed. This is proven by comparing all other 35 stations of our Dogger bank dataset. Over the 30-year period, 55 % of the species at stations 36 and 40 were similar. In the future, the stations should be sampled equally often to ensure an optimal comparison. Additionally, fishing will presumably not stop at the Dutch sector while it will stop at the German sector. Also, Shannon-Wiener indices as well as the most abundant species did not differ between the two stations. The analysis of the benthic fauna in the surrounding stations of Station 40 during the sampling in 2021 showed that the species composition is relatively uniform, while biodiversity differs due to different mesh sizes, as shown in the results section. Additionally, stations 1 and 13 are somewhat out of line in the evaluation, which can be explained by the fact that these stations are located at the edge of the study area and thus also at the edge of the Dogger Bank. The remaining stations show very similar benthic species composition, which means that the sampling of a permanent station can certainly be used as a representative of trend developments at a certain distance.

Figure 6 documents that more species were present at station 40. However, both curves showed that there always was a surge of new species. The increase in the number of species found over the years can be explained by the fact that it takes a certain number of sampling runs to capture approximately all species in a locality, since not all species occur at the same time during the day at one location. This also explains why the biodiversity differs in the MDS plot according to Bray-Curtis. Since the reference station records diversity only once per sampling, it is apparently less diverse than that of station 40, which was sampled several times per season. Here, species with diurnal rhythms are also recorded with higher abundances, which allows a broader spectrum of benthic species to be represented during the course of the day. In comparison, the reference station thus appears less diverse, although this could be revised by multiple sampling runs as at station 40.

Since 2012, the annual sampling of station 40 with the RV “Senckenberg” is regrettably diminishing due to cancelled expeditions and/or bad weather conditions. It would be important to resume regular sampling so that future changes in benthos in the area can be compared to historical records. The biodiversity evaluation indicated that change is expected again in the next few years. To be able to study this change annual surveys of the area are essential. Regular sampling of the station at 48 hours with 3-hour intervals would also be interesting in the coming years to compare diurnal changes in benthos with the already existing data sets. Since an exclusion of mobile, bottom-dwelling fisheries is planned around station 40 in the coming years, but reference station 36 is located outside this area, a regular approach to both stations is important to keep an eye on the different abiotic parameters and their influence on the benthos, as mentioned above.

Nevertheless, a long-term study over this period is so far unique for the Dogger Bank. However, what becomes clear at both stations is the difference in benthic biodiversity over the years. Both fishing pressure and temperature changed over the years, both of which could have had an impact on the benthic fauna. To check which of the two factors has a stronger influence, a Principal Component Analysis (PCA) can be made (Granato et al. 2018; fig 13). This shows that all in all, both water temperature at depths below 30 m and fishing seem to have equal effects on the benthic fauna. Moreover, it is clear that fishing has a greater influence than temperature at both stations in the years 1991-2002, but this shifts in subsequent years. While fishing intensity decreases from 2003 onwards (fig. 2), water temperature continues to increase over the years (fig. 4). This is reflected in the PCA at both stations studied (fig. 13). In the years 2003-2021, temperature has a greater effect on the occurrence of benthic fauna. The effects of the two parameters are discussed in more detail below.

The influence of increased fishing effort on one hand appears to be negative for the abundance of some species, but on the other hand seems to be positive for the abundance of others, like scavengers. Asterias rubens and Pagurus bernhardus for example are both species that occur in higher numbers of individuals with increased trawling activity. Both of them do not belong to the exploited species and at most are affected by the fishery as bycatch. This supports the thesis that fisheries have an impact at the community level (Heino & Godø, 2002). Depending on the target fish, fishermen use larger mesh sizes in order to reduce the amount of bycatch (Glass 2000). Small species can therefore escape through the meshes of the nets and find a swirled layer of sediment (Riemann & Hoffmann 1991) that makes food easy to catch. A. rubens for example might feed on mussels brought to the surface from the top layer of sediment and may be able to open and eat them more easily because of damage to their shells (Mensink et al. 2000). P. bernhardus is significantly more abundant immediately after an area is fished, and stomach contents are also significantly higher in volume (Ramsay et al. 1996). Mobile species may draw an advantage from disturbance because they can quickly recolonize fished areas (Tulp et al. 2020). However, although some species may receive a temporary positive effect from bottom-touching fisheries, the negative consequences are predominant and cause ecosystem productivity to decline (Coleman & Williams 2002; Kaiser et al. 2002). The two flatfishes B. luteum and A. laterna fall into the fishermen's prey pattern (Cheung & Oyinlola, 2018), making it difficult for them to escape from the nets and thus are negatively affected by increased trawling activity (Audzijonyte et al. 2013). This also causes the mean size of intensively fished species to decrease on average, which can manifest evolutionary changes such as earlier sexual maturity (Johnson et al. 2015). However, these species are ultimately missing as food sources or predators of other species to keep food webs and energy flows in marine ecosystems in balance. In the German Wadden Sea, a shift of species has been noted already, positive for polychaetes and negative for molluscs and crustaceans (de Groot 1984). The aforementioned stirring up of sediment by bottom trawling also has a crucial disadvantage as the pollutants contained in the sediment are also stirred up and float freely in the near-bottom nepheloid layer, where the benthic fauna lives (Bradshaw et al. 2012).

Comparing fishing intensity in the entire German EEZ of the North Sea (fig. 14) it can be seen that intenseness is highest in the regions of the Wadden Sea. The Dogger Bank, which is located at the north-western end of the EEZ, shows a lower intensity. Transferring the results of this study to the much more intensively fished areas of the North Sea, even more drastic changes can be expected with regard to the epifauna. Several studies confirm that epifauna communities in the Wadden Sea change over the years (Reise & Schubert 1987; Buhs & Reise 1997).

Regular fishing of a certain area shows bigger effects on ecosystems (Jones 1992). Therefore, limiting bottom-contact fishing in designated zones is useful to be able to ensure the rehabilitation of marine ecosystems and their benthic fauna. It is important that periods between fishing are sufficiently long, as studies have already shown that recovery in benthic fauna was not seen at fished sites even after one year (Cook et al. 2013).

In addition to the direct impact of bottom contact fishing, trawlers also have indirect effects on the behavior of various commercial fish species, for example by activating escape behavior (Heino & Godø, 2002). In a second step, this could lead to behavioral adaptations and thus affect fish stocks by causing unnecessary escape reactions to lead to a wrong distribution of resources among individuals (Heino & Godø, 2002). Thus, a complete exclusion of mobile bottom-dwelling fisheries in certain protected areas in the North Sea would be more advisable than just a reduction. In order to achieve a state of ecosystem-friendly fisheries in general, there have been studies so far on indicators regarding ecosystem status, fishing pressure or even management progress (Jennings 2005). The regulation on discarding bycatch as well as the use of selective nets should also be reviewed in consultation with fishermen (Catchpole et al. 2005).

In addition to fishing, climate change is also affecting benthic species in marine ecosystems (Birchenough et al. 2015). A study of long-term data from the North Sea from 1978 to after 1987 already shows that fishing has an effect on fish stocks, but that changes in environmental conditions could exacerbate this effect (Reid & Edwards 2001). Advancing climate change is resulting in an increase in water temperature (Bijma et al. 2013), which is not stopping at the North Sea. This can be seen in the evaluation of the water temperatures of the summer months, showing not only surface temperatures to increase, but also deeper layers experiencing a remarkable warming. However, the benthic fauna in the more northern areas of the North Sea with a greater water depth yet only some species seem to be affected, in contrast to the shallow areas of the southern North Sea (Neumann et al. 2009). In general, the effects of elevated temperature on species depend on their specific temperature sensitivity (Freitas et al. 2007). However, climate change-induced introductions of benthic, invasive species such as the angular crab Goneplax rhomboides (Linnaeus, 1758) are causing that areas need to be repopulated and ecosystems restructured due to overlapping niches (Neumann et al. 2013) and species range shifts occur (Lord 2014). A long-term study of epifauna communities in the North Sea showed that reproduction in particular is affected by temperature anomalies (Neumann & Kröncke 2011). However, the Dogger Bank is an elevation in the North Sea with comparatively shallow water depths, so temperature changes could have a stronger impact on epifauna communities in that region compared to others. As bottom-touching fishes move to deeper areas in response to temperature change (Dulvy et al. 2008), the shallow areas of the North Sea could expect to lose species richness. Deeper areas should be protected accordingly.

Long-term studies such as these are now and will continue to become increasingly important as climate change and other environmental factors continue to impact benthic biodiversity (Clark et al. 2003) and to understand (Neumann et al. 2013) and develop management plans (Ducrotoy et al. 2000). Because benthic fauna is, in part, an important food source for commercial fish species, climate change and fisheries significantly influence each other (Hiddink et al. 2014). Epifauna communities have a recovery time of about 7-8 years (Hermann & Kröncke 2011), which means that possible protection measures in any respect should be started as soon as possible to minimize irreparable damage to ecosystems.

Acknowledgements

We thank the Senckenberg Research Institute and Natural History Museum for the opportunity to carry out the MGF North Sea project. Furthermore, we thank the crew of the F.K. "Senckenberg" for the always safe sailing on the North Sea. We also thank Prof. Dr. Ingrid Kröncke for coordinating the project. Thank is also due to all helpers who assisted in the identification of numerous species. Many thanks to Pascal Hillgruber for the check regarding the English writing style. Sven Tränkner kindly provided the species images. to the time and efforts of the reviewers and editors of the journal as well as that of the anonymous reviewers is much appreciated. The BMBF supported the project (BMBF grant 03F0847D).

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Climate change and fisheries affect benthic composition and diversity in the North Sea - Investigations at the Dogger Bank during three decades (1991-2021)

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