Fjords are disproportionately important for global organic carbon (OC) burial relative to their spatial extent and may be important in sequestering atmospheric CO2, providing a negative climate feedback. Within fjords, multiple locally variable delivery mechanisms control mineral sediment deposition, which in turn modulates OC burial. Sediment and OC sources in Fiordland, New Zealand, include terrigenous input at fjord heads, sediment reworking over fjord-mouth sills, and landslide events from steep fjord walls. Box cores were analyzed for sedimentary texture, sediment accumulation rate, and OC content to evaluate the relative importance of each delivery mechanism. Sediment accumulation was up to 3.4 mm/yr in proximal and distal fjord areas, with lower rates in medial reaches. X-radiograph and Pb-210 stratigraphy indicate mass wasting and surface-sediment bioturbation throughout the fjords. Sediment accumulation rates are inversely correlated with %OC. Spatial heterogeneity in sediment depositional processes and rates is important when evaluating OC burial within fjords.

Phytoplankton pigments in sediment cores from four New Zealand fjords were quantified to investigate community composition and primary production in this pristine and remote region. Downcore sediment records from Doubtful Sound, Fiordland, were also compared with phytoplankton pigments in sediment traps to investigate pathways of phytodetritus flux from the upper water column to the sediment. Historic primary production was estimated using downcore chlorophyll a (Chl a) and β-carotene as proxies for total algal biomass. Sedimentary Chl a was similar across Fiordland (p = 0.09), but β-carotene was significantly different in Broughton Arm (Kruskal–Wallis p < 0.01). Dominant transformation products of Chl a were steryl chlorin esters and carotenol chlorin esters, indicating the importance of grazing as a sink for phytoplankton in Fiordland. Carotenoids indicative of diatoms and dinoflagellates were recovered in the sedimentary records of Doubtful Sound. Pigment biomarkers were observed in higher quantities in the sediment traps than in the surface sediment. In particular, grazing biomarkers were abundant in the sediment, highlighting the importance of fecal pellet export in phytodetritus preservation. Phytoplankton pigment preservation in Fiordland sediments is good as indicated by little variation in Chl a/pheopigment ratios throughout the cores. This research supports other new work, which has shown that carbon preservation in southern hemisphere fjord ecosystems is more efficient than previously thought.

Doubtful Sound, New Zealand, provides an exceptional opportunity to study a ‘baseline’ coastal ecosystem with an intact watershed. We present the first data on historical changes in phytoplankton abundance and community composition for three sites in Doubtful Sound using sediment records. Profiles of sedimentary concentrations of β-carotene (a proxy of total algal abundance, 0.021–1.345 mmol g organic carbon−1) and carotenoids were generally depleted, indicating low autochthonous production. Phytoplankton pigments and diatom frustules in Doubtful Sound indicate that diatoms have been prevalent for at least the last ca. 350 years; however, the relative importance of marine and freshwater diatoms has varied through time. Further, the timing of change in phytoplankton biomass and community composition differed among the sites within Doubtful Sound. This finding highlights the need to use multiple sites and complementary biomarkers when studying historical changes in phytoplankton communities in complex ecosystems with strong physicochemical gradients such as fjords.

Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin-and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux.