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Abstract

Mangroves and seagrass meadows create coastal seascapes acting as breeding and nursery grounds for many fish species in tropical areas. Despite increased awareness on the role of these coastal habitats to fish, less knowledge is given to fish larvae distribution and population dynamics, limiting the understanding of major drivers of fish population dynamics and fisheries resources management, particularly in understudied regions such as the Western Indian Ocean. This thesis aimed at understanding seasonal and spatial patterns of fish larvae populations and community composition in relation to key environmental variables and spatiotemporal patterns of zooplankton in mangrove-seagrass seascapes of coastal East Africa. To address the specific goals of the thesis, four studies were performed and presented (Papers I-IV). Paper I focused on seasonal patterns of fish larvae distribution across mangrove-seagrass seascapes at the scale of Zanzibar (Tanzania). In Paper II, we studied seasonal variability of fish larvae distribution  and association to environmental characteristics in seagrass meadows (at the scale of East Africa). In Paper III, we investigated seasonal patterns of zooplankton abundance and community composition across mangrove-seagrass seascapes of Zanzibar. Paper IV addressed how zooplankton community composition and water physicochemical properties influence fish larvae abundances across monsoon seasons in mangrove-seagrass seascapes of Zanzibar. The findings of Paper I showed that abundance and richness of fish larvae varied across different months and among habitats (mangrove creeks, nearshore seagrass meadows and inshore seagrass meadows), whereas there were no significant differences between the southeast monsoon (SEM) and northeast monsoon (NEM) seasons. The abundance of fish larvae was particularly high during the SEM season in mangrove creeks at one of the two studied seascapes. Assemblage composition of fish larvae did not change over time in any habitat. The findings suggest that mangroves and seagrass meadows are connected systems with similarity in assemblage compositions. Paper II displayed significant seasonal variations in abundance and community composition of fish larvae in two of the studied areas. It also showed site-specific influences of temperature, salinity, chlorophyll a and current velocities, on fish larvae abundance. Paper III showed  that zooplankton abundance and community composition strongly varied across the two monsoon seasons and among habitats,
with a significant difference between mangrove creeks and two different seagrass habitats. There was little influence of key environmental parameters during both monsoon seasons, with the exception of salinity (during precipitation peaks in SEM) and chlorophyll a, which influenced zooplankton abundance in mangrove creeks (in the Chwaka Bay seascape). The findings suggest that local hydrodynamics and seasonal variations have strong influences on zooplankton abundance and community composition in mangrove-seagrass seascapes. Paper IV revealed that fish larval abundances and diversity were generally strongly influenced by salinity and water temperature. In the Chwaka Bay seascape, fish larvae abundances were positively associated with abundances of copepods and invertebrate larvae groups of zooplankton in both monsoons and
all three habitats. In the Fumba seascape, fish larvae abundances were strongly (positively) associated with temperature, chlorophyll a and zooplankton diversity, and to some degree also (negatively) related to abundances of non-copepod zooplankton (e.g cirripeds larvae). This thesis comprehends essential information on how fish larvae vary in time and space of tropical coastal seascapes, and how these patterns relate to spatiotemporal distributions of zooplankton and contemporary environmental conditions. Area-specific information integrating the monsoonal influence and spatiotemporal dynamics should be considered in conservation efforts of fish larvae and their habitats in coastal East Africa.