Phytoplankton Dynamics in a Cultivated Kelp Farm off the coast of Namibia

Abstract

The rapid degradation of ocean habitats in the Anthropocene has significantly impacted marine ecosystems and fisheries, necessitating restoration efforts and sustainable practices such as the cultivation of Macrocystis pyrifera (giant kelp). This study examines the ecological impact of giant kelp farming on phytoplankton dynamics within the Benguela Current off the coast of Lüderitz, Namibia. Over two years (2022-2024), we monitored phytoplankton abundance, community composition across environmental gradients, and changes in alpha diversity indices across 5 adjacent sites at the surface, middle and bottom of the water column. Initial data exploration revealed seasonal peaks in phytoplankton cell counts. 26 different genera were found to exhibit variability in total cell counts across different sites and depths. Notably, Shearwater Bay (site of kelp farm) showed the highest genera diversity with similar cell count across all genera. Conversely, Grosse Bucht (Control) exhibited the lowest number of genera and the lowest cell counts. Depth-specific trends indicated that surface levels (0 meters) consistently hosted all genera at the highest cell count, The cell count as well as number of genera gradually decreased deeper in the water column (15-, 30-meters depth). A large presence of genus Chaetoceros was found at Pilot and in the surface waters. The lowest presence was recorded at Shearwater Bay’s surface waters. Additionally, cell abundance seemed to be correlated with the availability of nutrients Non-metric multidimensional scaling (NMDS) plots and PERMANOVA analyses indicated that community composition did not significantly vary by site or depth but was significantly influenced by CaCO3 availability, particularly during winter and autumn. Alpha diversity indices (Shannon, Simpson, and Inverse Simpson) demonstrated varying trends across sites and depths. Sites with cultivated kelp showed varied trends in Shannon diversity from 2022-2024, with significant increasing trends observed at Pilot, particularly at surface waters. Temporal trends in the Simpson and Inverse Simpson indices showed slight but non-significant changes. Overall, the findings suggest that giant kelp farming influences phytoplankton community dynamics. These changes underscore the ecological role of kelp farming in modifying biodiversity and its potential downstream impacts on marine ecosystems. Further long-term studies are needed to elucidate the broader implications of kelp cultivation on marine biodiversity and ecosystem functioning, particularly in light of ongoing anthropogenic pressures on the atmosphere and ecosystems alike.