Small-scale variability in geomorphological settings influences mangrove-derived organic matter export in a tropical bay

Abstract

Abstract. Organic matter (OM) exchanges between adjacent habitats affect the dynamics and functioning of coastal systems, as well as the role of the different primary producers as energy and nutrient sources in food webs. Elemental (C, N, C : N) and isotope (δ 13 C) signatures and fatty acid (FA) profiles were used to assess the influence of geomorphological setting in two climatic seasons on the export and fate of mangrove OM across a tidally influenced tropical area, Gazi Bay (Kenya). The main results indicate that tidal transport, along with riverine runoff, plays a significant role in the distribution of mangrove organic matter. In particular, a marked spatial variability in the export of organic matter from mangroves to adjacent habitats was due to the different settings of the creeks flowing into the bay. Kinondo Creek acted as a mangrove retention site, where export of mangrove material was limited to the contiguous intertidal area, while Kidogoweni Creek acted as a "flow-through" system, from which mangrove material spreads into the bay, especially in the rainy season. This pattern was evident from the isotopic signature of primary producers, which were more 13 C-depleted in the Kinondo Creek and nearby, due to the lower dilution of the dissolved inorganic carbon (DIC) pool, typically depleted as an effect of intense mangrove mineralisation. Despite the trapping efficiency of the seagrass canopy, suspended particulate OM showed the important contribution of mangroves across the whole bay, up to the coral reef, as an effect of the strong ebb tide. Overall, mixing model outcomes indicated a widespread mixed contribution of both allochthonous and autochthonous OM sources across Gazi Bay. Moreover, FAs indicated a notable contribution of brown macroalgae and bacteria in both sediment and suspended pools. These results suggest that ecological connectivity in Gazi Bay is strongly influenced by geomorphological setting, which may have far-reaching consequences for the functioning of the whole ecosystem and the local food webs.

Species Traits and Geomorphic Setting as Drivers of Global Soil Carbon Stocks in Seagrass Meadows

“…6A). The association of lagoons and tidal systems with riverine inputs can lead to a dominant contribution of allochthonous carbon to these systems (Signa et al, 2017;Watanabe & Kuwae, 2015), which may partly explain the higher OC stocks found in these environments (Fig 2B), and is coherent with the trend of more positive Δ 13 C values (more dominated by allochthonous C) found, particularly, in lagoons (Fig. 4B).…”

Participatory forestry improves mangrove forest management in Kenya

“…Two seasonal rivers -Kidogoweni to the northwest and Mkurumudji to the southwest of the bay drains into the Bay. This is in addition to seepage points (Signa et al 2017) that provide the Bay with freshwater input. According to Koppen climate classi cation, the climate of Gazi Bay may be classi ed as tropical wet/dry that is ideal for supporting mangrove growth.…”

“…This management block is located 3-5 km from Makongeni village (Fig. 1) and has no direct freshwater in uence (Signa et al 2017). The eastern block is largely managed by Kenya Forest Service (KFS), a state forest agency in charge of forests in Kenya, and there is minimal community involvement.…”

Horizontal and vertical food web structure drives trace element trophic transfer in Terra Nova Bay, Antarctica

“…Identification of principal OM pathways of the food web was carried out using a qualitative approach. Differences in δ 13 C values have been shown to provide information on the contribution of sympagic vs. benthic and pelagic sources to the diet of Antarctic consumers (Calizza et al, 2018;Norkko et al, 2007), reliably reflecting differences in OM pathways between organisms (Careddu et al, 2015;Rossi et al, 2015;Signa et al, 2017b). Similarly, δ 15 N values are known to increase unequivocally across trophic levels, and therefore, provide a robust base to identify trophic position of consumers within food webs (Mancinelli et al, 2013;Post, 2002) The effect of the horizontal and vertical food web structure (i.e.…”