Heraklion BBT covers the entire bay of Heraklion also including Dia Island and its coasts, which is located 10 km offshore from the coast of Heraklion City. Heraklion Bay consists mainly of soft-bottom habitats (sand and mud) and reaches depths exceeding 200 m. However, roughly half of the coastline of the Heraklion BBT is rocky rugged terrain that drops into the sea forming Mediterranean rocky reefs which can be substantial locally. Typical eastern Mediterranean conditions prevail in the Bay, with primary productivity being low (80 gC/m²/year) and the area characterised as oligotrophic (Psarra et al. 2000), relatively unaffected by freshwater run-off from rivers or wastewater discharges (Koukounari et al. 2020).

Several studies have investigated the macrobenthic faunal communities of the soft-bottom habitats of Heraklion Bay (Karakassis and Eleftheriou 1997, Plaiti et al. 2000, Tselepides et al. 2000, Koulouri et al. 2006, Koulouri et al. 2013, Tsikopoulou et al. 2019). However, these studies have been sporadic and long-term data are limited, restricting our understanding of temporal ecological shifts that could be affecting those habitats due to environmental and human-induced disturbances and especially climate change. Additionally, established non-indigenous fish and benthic species are found in large numbers in the Bay (Katsanevakis 2020, Ragkousis 2023), exerting pressure in a variety of habitats. The two Siganid species of rabbitfish have overgrazed (for more than two decades) the rocky reefs of the Bay leading to the shift from macroalgae-dominated reefs to barren reefs. In addition, other non-indigenous grazers like Diadema setosum further increase the overgrazing pressure on the rocky reefs. New species are continuously reported in Heraklion Bay and together with the rising water temperature are contributing to the pressure on the habitat of the area through tropicalisation (Zarzyczny et al. 2024).

To bridge the gap between research, policy and practice, the project is focused on stakeholders whose experiences and interests can be considered and represented in the decision-making tools and instruments. To this end, representatives from a range of different sectors (public audience, public authorities, industry/private sector, academia/research) have been invited to become involved in MARBEFES and share their point of view regarding Heraklion BBT. Between 3 and 7 April 2023, interviews and surveys were conducted with 18 different types of stakeholders (30 individuals) from the four above-mentioned stakeholder domains (e.g. museums, aquaria, research centres, Region of Crete, Ministry of Environment, NGOs, municipalities, cruise sector, commerce, tour guides, divers, teachers, hotel owners) related to the marine research area of Heraklion Bay (BBT12). According to the stakeholders of Heraklion BBT, the coastal development on the northern shoreline is very high and, therefore, is considered a pressure on the system. Control of the pressures and risks stemming from the coastal development should be much more improved, while there is a need for stronger governance to ensure sustainable development of the economic sectors. Mass-tourism is also strongly present across the northern shore, accompanying often by negative connotations.

In this study, three main events that lead to significant biodiversity change are considered in a bow-tie risk analysis (Fig. 1). These events are:

the barren rocky reefs;

On the left-hand side of the bow-tie analysis, under causes, lies the list of activities affecting the marine environment and anthropogenic pressures through which human activities affect the marine ecosystem of the BBT. Preventative controls follow, aiming to reduce the aforementioned pressures. On the right-hand side of the bow-tie figure, we present the mitigation measures (controls) and the consequences of biodiversity change. To implement the tool across European seas, the standardised vocabulary and classification of each element is applied, following the methodology described by Elliot et al. (2025).

Letters and numbers in the text and Fig. 1 are decoded as following: Ctrl3.1 - Monitoring (of the environment, activities, impacts on the ecosystem and/or society), Ctrl4.1 - EU/international legislation (enforcement, new adoption), Ctrl4.2 - National/local legislation (enforcement, new adoption), Ctrl4.1.2 - Adaptive legislation which can react to unexpected positive or negative changes, allow innovation development, Ctrl5.1.1 - Government compensation payments to, for example, mitigate the loss of revenues and Ctrl5.1.2 - Economic incentives to improve/promote/support alternative activities with lower impact on biodiversity (innovation).

The causes of biodiversity change in Heraklion Bay have been identified through expert judgement and consultation with stakeholders including museums, aquaria, research centres, Region of Crete, Ministry of Environment, NGOs, municipalities, cruise sector, commerce, tour guides, divers, teachers and hotel owners. The central events described in the bow-tie analysis (Fig. 1) are:

the shift to barren rocky reefs;

Barren rocky reefs. Input or spread of non-indigenous species (P1.1) is the main pressure driving the once macroalgae-dominated Mediterranean rocky reefs of the area to become barren reefs. The two non-indigenous species of rabbitfish Siganus luridus and Siganus rivulatus which have been established in Heraklion Bay for decades now and aggregate in impressive numbers are overgrazing the canopy of the rocky reefs to such an extent that only turf algae can survive on them. Additionally, the non-indigenous urchin Diadema setosum has also established in the area in great numbers, further increasing the overgrazing pressure on the rocky reefs. The latter, having lost the intricate three-dimentional complexity that the macroalgal canopy provides, are becoming less diverse, leading to decrease in provisioning (C1.2) and regulating (C1.3) Ecosystem Services. This could further enhance the risk of even more non-indigenous species establishing and further altering their biodiversity. There is also a decrease of aesthetic value/benefits (C3.1) associated with the dominance of barren reefs. Apart from continuously monitoring (Ctrl3.1) the ecological state of the rocky reef habitat, ecosystem restoration (Ctrl1.2.1) can be utilised to mitigate this phenomenon.

Non-indigenous species. Except for the aforementioned non-indigenous species, more have established in Heraklion Bay as a result of the pressure that climate change (P4) exerts on the marine environment. Some of them, like Lagocephalus sceleratus, have negative effects as they interfere with commercial activities like fishing. Others, like Torquigener flavimaculosus and Parupeneus forsskali, are very abundant in Heraklion Bay and, by direct competition, are contributing to the reduction of commercially important native species, for example, Mullidae. The result is, like previously, a decrease in provisioning (C1.2) and regulating (C1.3) Ecosystem Services and also a change in the underwater seascape (C1.4) and decrease in other cultural benefits (C3.4). Once again, continuously monitoring (Ctrl3.1) the ecological state of the marine environment is essential and ecosystem restoration (Ctrl1.2.1) can be utilised to mitigate this phenomenon.

Loss of native biodiversity. All of the pressures shown in the bow-tie analysis (Fig. 1) are linked to the decrease/loss of native biodiversity. More specifically, human impacts (litter, sewage, emissions) (P3.4 and P3.2) and the disturbance caused by anthropogenic sound (P3.5) are the pressures exerted by Tourism and Leisure (A8) through infrastructure (A8.1) and activities (A8.2). Extraction of wild species (P1.6) from the marine environment and the Physical Disturbance to seabed (P2.1) are the pressures linked to the Extraction of living resources (A4) and, more specifically, to fish and shellfish harvesting (A4.1). Last, but not least, the spread of non-indigenous species (P1.1) and climate change (P4) are also contributing to loss of native biodiversity. However, their causes cannot be found in the strict confines of Heraklion BBT as they are phenomena concerning the whole of the Mediterranean and beyond and are linked to global trade and decision-making/politics. The consequences of the loss of native biodiversity are very serious since not only do we have a decrease in provisioning (C1.2) and regulating (C1.3) Ecosystem Services, but also direct economical consequences on groups like fishers by decreasing their income (C2.2) and having a negative impact on the labour market (C2.4). Mitigation measures for this are government compensations (Ctrl5.1.1) for loss of income and economic incentives to promote innovative activities with lower impact on biodiversity (Ctrl5.1.2). Once again, continuously monitoring (Ctrl3.1) of the ecological state of the marine environment is essential and ecosystem restoration (Ctrl1.2.1) can be utilised to mitigate the loss of native biodiversity.

Climate change has a great effect on the marine environment of the area and, since it is a global phenomenon, its causes cannot be confined to the study area. It is unknown if the establishment of barren reefs, the increase of non-indigenous species and the tropicalisation of most of the eastern Mediterranean is reversible, but continuous monitoring along with an effort to mitigate the effects of climate change are critical. Large-scale tourism has also been identified as an important driver of biodiversity change in Heraklion BBT, further enhancing the pressure on the marine environment. Therefore, monitoring of macrobenthic communities of soft-bottom habitats is crucial as they are considered to be one of the main Biological Quality Elements (BQEs) used under the Water Framework Directive to assess ecological status in coastal waters, as well as being sensitive indicators of environmental changes and human impacts under the Marine Strategy Framework Directive. Touristic development needs to be regulated, as overexploitation of resources and the associated physical disturbance are enhancing the degradation of the marine environment of Heraklion Bay.