Harnessing the power of Squidle <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg" display="inline" id="d1e3745"> <mml:mo>+</mml:mo> </mml:math> to develop flexible machine learning models

While our ability to capture seafloor imagery has expanded dramatically, extracting information remains a major bottleneck, as annotation is still largely manual, time-consuming, and costly. Machine Learning (ML) classifiers offer a promising solution, but they often lack generalisation as they are typically trained on small, homogeneous datasets from selected campaigns. Here, we address this challenge by leveraging the Squidle+ platform to assemble a heterogeneous dataset of 1.7 million annotations across 150,000 images from 325 expeditions in Australian waters. To effectively train models on this scale, we introduce a novel dataset distribution strategy, the neighbour-distribution, and implement adaptive cropping to account for varying image resolutions. To assess performance at a national scale, we excluded entire campaigns from training and used them as independent test sets, allowing evaluation on complete transects, in addition to composed datasets. Using this flexible training pipeline, we combined diverse annotation schemes into three high-level Essential Ocean Variables (EOVs): Seagrass, Canopy-Forming Macroalgae, and Hard Coral cover and one species-specific label, Ecklonia radiata. This represents the first use of such a large and diverse dataset for training an ML classifier of Ecklonia radiata. The resulting models achieved test accuracies of 85%-93% for the EOVs and 96.5% for Ecklonia radiata on campaign test sets spanning Australia's coastline, demonstrating robust performance at a national scale. To enhance accessibility, both the code and trained models are made publicly available and directly integrated into Squidle+, lowering the barrier for their use in ecological monitoring programs.