Looking at past phosphorus accumulation in a Florida lake offers new insight on recent cultural nutrient enrichment

A Holocene Sediment Record of Phosphorus Accumulation in Shallow Lake Harris, Florida (USA) Offers New Perspectives on Recent Cultural Eutrophication

by: William F. Kenney, Mark Brenner, Jason H. Curtis, T. Elliott Arnold, Claire L. Schelske

Summarized by: Mckenna Dyjak

What data were used?: A 5.9 m sediment core was taken in Lake Harris, Florida using a piston corer (a technique used to take sediment samples, similar to how an apple is cored). Lake Harris is a subtropical, shallow, eutrophic body of water (rich with nutrients) located near Orlando, Florida.  

Methods: The 1.2 m sediment core is long enough to provide the complete environmental history of Lake Harris. However, the core must be interpreted first. In order to do so, the core was first dated using lead isotope 210Pb and carbon isotope 14C. The next steps involved using proxy data (preserved physical characteristics of the environment) to determine net primary productivity (the concentration and accumulation rates of organic matter), lake phosphorus enrichment (three forms of phosphorus), groundwater input (concentration and accumulation rates of carbonate material, like limestone), macrophyte abundance (e.g., sponge spicules), and phytoplankton abundance (e.g.,diatoms).

Results: The study found that Lake Harris began to fill with water in the early Holocene (~10,680 calendar years before the present) and transitioned to a wetter climate in the middle Holocene. The transition is indicated by a change in carbonate to organic sediments; a higher amount of organic sediments would suggest an increase in rainfall needed to support the plant life that would become the organic matter. A low sedimentation rate indicates that the lake was experiencing oligotrophication (depletion in nutrients) through the Holocene until around the 1900s. After the 1900s, there were increased sedimentation rates (Figure 1. A, B, D, and E) which indicates cultural eutrophication (increase of nutrients in bodies of water). Phosphates and nitrates from common fertilizers and other human activities (which is why it’s called “cultural eutrophication”) can allow algae (e.g., diatoms) to grow rapidly and reduce the amount of oxygen in the lake. An increased sedimentation rate can be used to determine whether a body of water is in a state of eutrophication, because the amount of phytoplankton (such as diatoms) would increase in accumulation. Total phosphorus accumulation rates can also indicate eutrophication.

Figure 1. Sedimentation rates for (A) bulk sediment, (B) organic matter, (C) CaCO3, (D) total phosphorus, (E) diatom biogenic silica and (F) sponge spicule biogenic silica versus core depth. Near the top of the core we can see a significant increase in A, B, D, E, and F which provide evidence for cultural eutrophication (increased sediment rates).

Why is this study important?: This study shows that, without being disturbed, Lake Harris was prone to becoming depleted in nutrients, the process of oligotrophication. The complete change of course due to human activities (i.e., fertilizer runoff) is more detrimental than was previously considered. This study showed that throughout the environmental history of Lake Harris there was never a sign of natural eutrophication, but rather that of oligotrophication. 

The bigger picture: Cultural eutrophication is a serious problem plaguing many aquatic systems and has serious consequences such as toxic algae blooms, which can have far reaching effects like on the tourism industry in Florida! The extent of damage caused by human activities is shown in this study and can help us understand how lakes responded in the past to the introduction of cultural eutrophication.  

Citation: Kenney WF, Brenner M, Curtis JH, Arnold TE, Schelske CL (2016) A Holocene Sediment Record of Phosphorus Accumulation in Shallow Lake Harris, Florida (USA) Offers New Perspectives on Recent Cultural Eutrophication. PLoS ONE 11(1): e0147331. https://doi.org/10.1371/journal.pone.0147331

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