Background
The authors write that the "increased flow of human-derived nutrients and effluent into estuaries can reduce water quality and promote growth of undesirable aquatic primary producers (e.g. phytoplankton and benthic microalgae), which can lead to changes in the composition of organic matter within sediments, particularly if aquatic plants such as seagrasses and mangroves become displaced (Carpenter et al., 1998)," and they say that this development "can significantly disrupt benthic communities that are a critical part of detritus-based food webs (e.g. Kelaher and Levinton, 2003), and will have serious consequences for the effectiveness of coastal systems as global carbon sinks, given the lower burial efficiencies of microalgae relative to seagrasses and C3 terrestrial plants (Mateo et al., 2003, 2010; Lovelock et al., 2010; Sanders et al., 2010)."

What was done
Macreadie et al. further explored this important subject by reconstructing "the sedimentary records of cores taken from two sites within Botany Bay, Sydney - the site of European settlement of Australia - to look for human-induced changes in dominant sources of detritus," where the cores "covered a period from the present day back to the middle Holocene (~6000 years) according to ²¹⁰Pb profiles and radiocarbon (¹⁴C) dating."

What was learned
The five Australian researchers found that "sedimentation rates in the last 30-50 years were considerably higher than during the rest of the Holocene," and that "C:N ratios declined and began to exhibit a microalgal source signature from around the time of European settlement, which could be explained by increased nutrient flows into the Bay caused by anthropogenic activity." In addition, they report that "analysis of stable isotopic ratios of ¹²C/¹³C showed that the relative contribution of seagrass and C3 terrestrial plants (mangroves, saltmarsh) to detritus declined around the time of rapid industrial expansion (~1950s), coinciding with an increase in the contribution of microalgal sources."

What it means
Quoting Macreadie et al., "we conclude that the relative contribution of microalgae to detritus has increased within Botany Bay, and that this shift is the sign of increased industrialization and concomitant eutrophication." And in light of "the lower carbon burial efficiencies of microalgae (~0.1%) relative to seagrasses and C3 terrestrial plants (up to 10%)," they say that "such changes represent a substantial weakening of the carbon sink potential of Botany Bay," adding that "this occurrence is likely to be common to human-impacted estuaries." Once again, therefore, we have another good example of the local water-polluting activities of man having a much greater impact on our immediate environment than our global air-enriching activities, which release CO2 into the atmosphere and thereby enhance the vegetative productivities and water-use efficiencies of earth's terrestrial plants.

References
Carpenter, S.R., Caraco, N.F., Correll, D.L., Howarth, R.W., Sharpley, A.N. and Smith, V.H. 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8: 559-568.

Kelaher, B.P. and Levinton, J.S. 2003. Variation in detrital enrichment causes spatio-temporal variation in soft-sediment assemblages. Marine Ecology Progress Series 261: 85-97.

Lovelock, C.E., Sorrell, B.K., Hancock, N., Hua, Q. and Swales, A. 2010. Mangrove forest and soil development on a rapidly accreting shore in New Zealand. Ecosystems 13: 437-454.

Mateo, M.A., Renom, P. and Michener, R.H. 2010. Long-term stability in the production of a NW Mediterranean Posidonia oceanica (L.) Delile meadow. Palaeogeography, Palaeoclimatology, Palaeoecology 291: 286-296.

Mateo, M.A., Sanchez-Lizaso, J.L. and Romero, J. 2003. Posidonia oceanica 'banquettes': a preliminary assessment of the relevance for meadow carbon and nutrients budget. Estuarine, Coastal and Shelf Science 56: 85-90.

Sanders, C.J., Smoak, J.M., Naidu, A.S. , Sanders, L.M. and Patchineelam, S.R. 2010. Organic carbon burial in a mangrove forest, margin and intertidal mud flat. Estuarine, Coastal and Shelf Science 90: 168-172.