Diversity of benthic diatoms inhabiting on/in the sediment of mangroves in Indonesia

Mangroves are known as highly productive areas, and 1/3 of primal carbon source of the ecosystem originates from mangrove tree litterfalls. This means remaining 2/3 is derived from other sources, including benthic diatoms richly inhabiting on/in the sediments (≈ microphytobethos/ MPB). However, our knowledges about MPB especially in mangrove areas are limited. In this study, MPB diversities were firstly investigated along the East Java coasts, Indonesia.

Field samplings were undertaken initially on Sep-Oct 2016 at three sites; Probolinggo, Situbondo, Banyuwangi, East Java, Indonesia. Total 27 benthic diatom species were found with an average of 97,723 ± 18,320 cells cm^-3. Microscopically- estimated total cell volume (biovolume) was 121 × 10⁶ ± 17 × 10⁶ µm³ cm^-3, equivalent to 1.03 ± 0.22 µg C cm^-3 following a volume-carbon estimation by Davies et al (2016). Considering the fact that muddy sediment of mangroves contains relatively low total organic content (<0.7%) (Alongi, 1988), the biomass of the benthic diatoms should not be neglected.

Second survey was done in Banyuwangi on March 2019, at where mangroves stretch along Pang-Pang Bay facing two distinct environments; intensive shrimp ponds at the west and a national forest at the east side. We found higher species diversity in the sediments than the previous survey, total 56 diatom species. Their density in the east area was higher than that of the west; 314,481 and 175,335 cells cm^-3, respectively. From total 28 stations data, there was a linear correlation (r²= 0.78) between photosynthetic activity (Fv/Fm) of the sediment surface and species diversity index (H’), indicating environment suitable for photosynthesis of benthic diatoms led higher diversity. In the following sampling at the site during high tide period, chlorophyll a of the water column was high in the inner part of the bay (17.5 µg l^-1), at where c.a. 60% of the phytoplankton population was dominated with MPB, probably suspended from the sediment by tidal movement.

This study denoted healthy mangrove forests with rich MPB and high photosynthetic activity. Considering the low carbon biomass, MPB might be insignificant as primary carbon sources for the benthic ecosystem; however, they could be behaving as primary producers in water column, those should be important faction in coastal fisheries.