ALBERT

Beschreibung: Seagrass beds provide a wealth of ecosystem services that benefit society (e.g., habitat and feeding ground for juvenile fisheries species), but the Anthropocene has seen to a global decline of these productive habitats. Many temperate estuaries are becoming eutrophic due to horticultural, agricultural, and urban nutrient run off, but the role of this enrichment in seagrass decline is not fully understood. In a multi-site manipulative field experiment (Tauranga Harbour, New Zealand; 37°S, 176°E), we elevated pore water nitrogen (N) concentrations (mimicking the consequences of long-term eutrophication) to examine effects on seagrass meadows. At six intertidal seagrass sites with differing sediment properties and macrofaunal communities, slow release urea fertiliser (200 g N m−2) was buried in 1 m2 plots at the start of the peak growing season (early summer). After 60 d, we measured several seagrass morphological variables (cover, leaf length and width, and above and below ground biomass), sediment properties, and macrofauna community structure. Results demonstrate that the resilience of seagrass meadows to N enrichment is highly site-dependent. Two of the six sites showed significant declines in a multivariate indicator of seagrass morphology, driven by marked reductions in seagrass cover and leaf length (of up to 78%). Whereas, other sites appeared resilient to N enrichment. It was expected that these site-specific responses would be correlated with changes in sediment properties that alter nutrient processing capacity (permeability and biogeochemistry). However, site-specific responses were instead correlated with the ambient seagrass biomass and macrofaunal diversity. Sites with low ambient seagrass biomass and macrofaunal diversity were less resilient to enrichment. These results highlight that seagrass biomass could be a good indicator of resilience to nutrient enrichment, and the biomass where resilience is lost may lie between 140 and 285 g DW m−2. This study contributes knowledge that is required for predicting and mitigating future impacts of estuarine eutrophication on seagrass ecosystems.

Beschreibung: Highlights • Novel multi-disciplinary approach to tracing freshwater and particle transport into boundary currents; • Significant glacial inputs reach coastal waters and are transported rapidly offshore; • Low surface water dissolved silicon concentrations maintained by diatom activity despite strong glacial and benthic supplies. Abstract Biogeochemical cycling in high-latitude regions has a disproportionate impact on global nutrient budgets. Here, we introduce a holistic, multi-disciplinary framework for elucidating the influence of glacial meltwaters, shelf currents, and biological production on biogeochemical cycling in high-latitude continental margins, with a focus on the silica cycle. Our findings highlight the impact of significant glacial discharge on nutrient supply to shelf and slope waters, as well as surface and benthic production in these regions, over a range of timescales from days to thousands of years. Whilst biological uptake in fjords and strong diatom activity in coastal waters maintains low dissolved silicon concentrations in surface waters, we find important but spatially heterogeneous additions of particulates into the system, which are transported rapidly away from the shore. We expect the glacially-derived particles – together with biogenic silica tests – to be cycled rapidly through shallow sediments, resulting in a strong benthic flux of dissolved silicon. Entrainment of this benthic silicon into boundary currents may supply an important source of this key nutrient into the Labrador Sea, and is also likely to recirculate back into the deep fjords inshore. This study illustrates how geochemical and oceanographic analyses can be used together to probe further into modern nutrient cycling in this region, as well as the palaeoclimatological approaches to investigating changes in glacial meltwater discharge through time, especially during periods of rapid climatic change in the Late Quaternary.