Treaty-Based Vs Voluntary Cooperation: Inter-Organisational Network Adaptation in CBSS and HELCOM After 2022

International organisations increasingly operate within dense networks of inter-institutional relationships, exchanging information and coordinating activities across overlapping mandates (Biermann & Koops, 2017; Franke, 2017). This paper examines the evolving network relationship between two prominent Baltic Sea Region organisations: the Council of the Baltic Sea States (CBSS) and the Helsinki Commission for the Protection of the Marine Environment (HELCOM), applying inter-organisational relations (IOR) theory to analyse how their interaction patterns have adapted to successive critical junctures from 1992 to 2024, with particular attention to post-2022 geopolitical crisis. CBSS, established in 1992 as an intergovernmental forum, and HELCOM, founded in 1974 as a convention-based environmental organisation, represent distinct organisational forms operating within the same geographic space. Despite overlapping memberships and complementary mandates in regional environmental governance, their network relationship has exhibited varying degrees of cooperation and competition. The analysis draws on documentary research, examination of organisational strategic documents, and semi-structured interviews with officials from both organisations conducted in 2024-2025, applying the multilevel IOR framework (Koops, 2017) and insights from organisational sociology. The study reveals three distinct phases in CBSS-HELCOM network relations. During the initial phase (1992-2004), characterised by post-Cold War optimism, organisations established complementary roles: CBSS launched Baltic 21 for sustainable development, whilst HELCOM maintained its specialised environmental mandate. This period witnessed cooperation and occasional tensions when CBSS initiatives overlapped with HELCOM's core competencies. The second phase (2004-2022), marked by Europeanisation through EU enlargement and the EU Strategy for the Baltic Sea Region (EUSBSR), saw both organisations becoming increasingly embedded in EU governance architecture, with HELCOM serving as an implementing mechanism for EU directives whilst CBSS coordinated multiple EUSBSR policy areas (Koivurova & Rosas, 2018). The research identifies complex patterns of network interaction, including formal strategic partnerships, operational cooperation through joint projects, knowledge exchange mechanisms, and coordination within the EUSBSR framework. Both organisations engage overlapping networks of national experts, scientific institutions, and NGO observers, creating dense horizontal linkages transcending formal organisational boundaries (VanDeveer, 2011). HELCOM particularly benefits from networked cooperation with OSPAR, IMO, and ICES for scientific knowledge production, whilst CBSS leverages networks with the Union of Baltic Cities and international organisations. The third phase (post-2022), following Russia's invasion of Ukraine, reveals divergent organisational responses. Following the suspension of cooperation with Russia in March 2022, Russia withdrew from CBSS in May 2022, fundamentally altering the organisation's membership. In contrast, whilst Russia remains a party to the Helsinki Convention and formally a HELCOM member, the organisation implemented a strategic pause while maintaining technical working group activities with modified participation. This asymmetric institutional disruption demonstrates differential network resilience mechanisms across organisations bound by different legal frameworks—voluntary intergovernmental cooperation versus treaty-based obligations—revealing how institutional design fundamentally shapes adaptation strategies and the limits of functional cooperation under severe geopolitical stress. The paper contributes to IOR scholarship by demonstrating how regional organisations navigate network relationships under changing conditions, how institutional complementarity and competition shape network effectiveness, and how geopolitical crises affect inter-organisational network stability, with network resilience fundamentally depending on the underlying political consensus.