Abstract

The East African Coastal Current (EACC) is the dominant oceanographic influence along the coastlines of Tanzania and Kenya yet formal descriptions of the biogeochemical characteristics of these waters remain fragmented or poorly defined. Whilst the region remains undersampled, and information for many parameters is limited or even absent, the region is not understudied and complex patterns, due in part to the changing monsoon seasons, can be identified from extant observations. A critical distinction between the neritic waters of the narrow East African continental shelf, which may be more influenced by local tidal currents and terrestrial inputs, and the oligotrophic surface waters of the deeper offshelf region under the influence of the EACC can be drawn, which cautions against the extrapolation of trends or seasonal patterns from limited datasets more widely throughout the region. Permanently N-limited, low NO3−:PO43− surface waters coupled with high (>25 °C) sea surface temperatures are a key feature of the EACC Ecoregion and likely responsible for the presence of a regionally important population of the nitrogen fixing cyanobacterium Trichodesmium, though information on another key requirement, iron, is lacking. Phytoplankton diversity, abundance and the spatiotemporal variability of phytoplankton populations are considered poorly known due to limited sampling efforts. Recent and growing recognition of high coral biodiversity, high reef fish species endemism, of widespread reductions in mangrove forest coverage, and growing anthropogenic pressures on coastal waters suggest that the region deserves greater multidisciplinary study. Efforts to anticipate climate induced changes to these waters, which are expected to impact local fisheries with substantial socioeconomic impacts, would benefit from greater efforts to synthesise existing biogeochemical data, much of which resides within grey literature sources, theses, project reports, remains inaccessible or has been lost. Future biogeochemical and oceanographic observational efforts should simultaneously explore shelf and deeper offshelf waters to determine shelf-to-ocean linkages and the spatiotemporal variability of parameter fields whilst also bridging the gap to research efforts on coral biodiversity, fisheries and marine management activities due to recognised gaps in underlying scientific data to support decision making in these areas.

Highlights

  • The tropical coastal waters of Tanzania and Kenya are bathed year-round by the northward flowing East African Coastal Current (EACC), a western boundary current of the Indian Ocean

  • No correlation between Pseudo-nitzschia spp, Pseudo1417 nitzschia pungens which was the most common species, and the measured variables of 1418 salinity, temperature, pH, dissolved oxygen, chlorophyll, NO3- or PO43- was identified. 1419 this study argues for awareness of the presence of a known toxic species but 1420 understanding the environmental controls on its abundance or distribution requires further 1421 work. 1422 1423 The extensive analysis of dinoflagelletes within the Indian Ocean reported by Taylor (1973; 1424 1976) identified over 300 species from 40 genera

  • The EACC Ecoregion is undersampled but not understudied – A rich picture can be drawn from the varied sources of information available

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Summary

Introduction

The tropical coastal waters of Tanzania and Kenya are bathed year-round by the northward flowing East African Coastal Current (EACC), a western boundary current of the Indian Ocean. Nyandwi (2013) noted that the maximum observed current speed within the Zanzibar Channel of 0.49 m s-1 was somewhat smaller than the maximum velocities of 1 to 2 m s-1 reported more generally for the EACC leading to the conclusion that no significant limb of the EACC funnels through the Zanzibar Channel This conclusion was recently verified following analysis of 24-years of surface drifter trajectories which indicated that the shallow water Mafia and Zanzibar Channels were not conduits for drifters and were not directly flushed by the EACC whereas the deep water Pemba Channel most certainly was (Semba et al, 2019). It is most likely that enhanced productivity over the outer shelf originates from oceanic influences, wind-driven or shelf-break upwelling

Pemba Channel
Deep Water
SE monsoon Intermonsoon
SE monsoon
Malindi and North
Annual Mean Annual cycle
Findings
1752 Conclusions

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