Research

Nutrient Ratios in Bermuda's Coastal Waters

Bermuda’s inshore waters have been traditionally considered pristine and not impacted by human activities. But quite to the contrary, ‘blooms’ of the benthic macrophyte Cladophora sp. in the 1970’s and more recently the near mono-specific, high biomass Pseudo-nitzschia pseudodelicatissima blooms have been interpreted as harbingers of eutrophication in Bermuda’s inshore waters. The benthic macroalgae Cladophora rapidly appeared in Harrington Sound in the mid-1970’s extinguishing an active Calico Clam fishery, and then disappeared as inexplicably as it appeared in the mid 1990’s. Roughly coincident in time with the disappearance of Cladophora sp. was the first ever-recorded major fish kill in Harrington Sound and a near mono-specific fall bloom of the diatom P. pseudodelicatissima. Extensive blooms of P. pseudodelicatissima have recurred during most fall blooms since this initial observation, most notably in 2002. There is no direct evidence linking the fish kill to the presence of P. pseudodelicatissima or production of the toxin domoic acid, but nonetheless they did occur at the same time and raise an interesting scientific question. Indeed, it is not our goal to explore a causal linkage between fish kills and P. pseudodelicastissima, but rather to explore the possibility that changes in nutrient inputs to Bermuda’s inshore bays have altered nutrient ratios (specifically Si:N and N:P) and in turn altered species successional patterns, possibly favoring growth of the diatom P. pseudodelicatissima.

Temporal Changes in Bermuda’s inshore Waters: Altered Si:N and Si:P ratios. Prior to the mid-1970’s the benthic macroalgae Cladophora sp. was not present in Harrington Sound and pelagic nitrate and nitrite concentrations averaged ~0.7 μM. During the Bermuda Inshore Waters Investigation (BIWI) study period, when Cladophora sp. was present in Harrington Sound, water clarity was quite high with Secchi depths averaging about 10-12m, pelagic chlorophyll (chla) concentrations were low and seasonally invariant, and nitrate and nitrite concentrations had decreased slightly (Figure 2). By the late 1980’s, when sampling resumed, water clarity had decreased nearly 30%, pelagic chla had increased slightly with little change in nitrate and nitrite concentrations (Figure 2); in 1995 Cladophora inexplicably disappeared from Harrington Sound. Coincident with this, was a large increase in pelagic chla through all of 1995 accompanied with increased seasonal variability in surface nitrate and nitrite concentrations. These increases in chla and nitrogen concentration also resulted in elevated dissolved oxygen and carbon dioxide concentrations supporting the importance of deep nutrient rich water inputs and a phytoplankton production response. The fall bloom in1995, following convective overturn and introduction of deep nutrient-rich waters to the euphotic zone was completely dominated by Pseudo-nitzschia pseudodelicatissima; the first recorded instance of such an event.

These data suggest that Cladophora may have been serving as a “cap” on the flux of nutrients through the benthic-pelagic interface and therefore “damping” benthic nutrient inputs. An alternate scenario might be that a significant increase in population density around Harrington Sound since the mid-1970’s has led to an increased importance of anthropogenic inputs. To some extent, both explanations are likely correct but mass balance calculations by BIWI investigators suggest that 35% of the nitrogen in Harrington Sound cycled through the benthos while only 5% came from external anthropogenic sources such as cesspits. The data collected after the BIWI program would suggest a shift from benthic-dominated to pelagic-dominated primary production, and an increase in the importance of P. pseudodelicatissima to phytoplankton dynamics in Harrington Sound. This scenario has been previously hypothesized as an important factor in the initiation and maintenance of the HAB species A. anophagefferens.

Contrary to the observations for nitrate and nitrite, inorganic phosphate levels have not increased since 1995 with the result being an overall decrease in the dissolved N:P ratio. One interpretation for the lack of increase in phosphate could be that the biological demand for phosphate is greater than the supply of phosphate. A significant concern here is that the increase in measured nitrate and nitrite will only force the Harrington Sound ecosystem towards P-limitation; a condition previously shown to induce toxin production in some Pseudo-nitzschia spp.. Supplies of silicate are governed by weathering of rocks and generally are not a component of anthropogenic nutrient inputs. This appears to be the case in Harrington Sound, as silicate concentrations have not varied between the 1970’s and the 1990’s. The consequences for Harrington Sound are that N:P ratios are increasing and Si:N ratios are decreasing. These altered nutrient ratios have significant impacts for at least two reasons. First, the absolute requirement for silicate in diatoms is associated with an increased requirement for phosphate, the result being hypothetically more efficient phosphate uptake mechanisms than in other phytoplankton. This hypothesis has been tested in marine phytoplankton and appears to be pervasive. Specific to Harrington Sound, diatoms are generally an important component of the phytoplankton assemblage, but the annual average contribution of diatoms to total phytoplankton biomass has increased between the 1970’s and the early-1990’s from ~20% (± 2.5%) to ~29% (± 2.9%). More importantly, observations of mono-specific diatom blooms appear to be more prevalent. Further shifts in the ecosystem towards P-limitation might further benefit diatoms. Second, the decrease in Si:N ratios would favor the growth of more lightly silicified diatoms such as P. pseudodelicatissima. Such has been shown to be the case in the Mississippi River plume where decreases in Si:N ratios has led to a relative increase in the importance of Pseudo-nitschia spp.

Collaborators on this research:

Drs. Pat Glibert and Todd Kana at Horn Point Laboratory - University of Maryland Center for Environmental Science

Dr. Debbie Bronk at the Virginia Institute of Marine Science

Publications Related to this Research:

Lomas, M. W., Glibert, P. M., Shiah, F-K., and Smith, E. M. 2002. Microbial Processes and Temperature in Chesapeake Bay: Current Relationships and Potential Impacts of Regional Warming. Global Change Biology 8:51-70. (download PDF)

Lomas, M. W., Trice, T. M., Glibert, P. M., Bronk, D. A. and McCarthy, J. J. 2002. Temporal and spatial dynamics of urea concentrations in Chesapeake Bay: Biological versus physical forcing. Estuaries 25:469-482. (download PDF)

Bronk, D.A., Lomas, M.W. Glibert, P.M., Schukert, K.J. and Sanderson, M.P. 2000. Total dissolved nitrogen analysis: Comparisons between the persulfate, UV, and high temperature oxidation methods. Mar Chem. 69:163-178. (download PDF)

Research Awards Supporting this Research:

This research was supported by National Science Foundation Awards BSR - 8814272 and OCE - 9521254

Other Relevant Research Links:

Chesapeake Bay Program Office

BBSR Environmental Quality Programs

Organic Nutrient Cycling in the Chesapeake Bay:

The importance of NO₃^- and NH₄⁺ to phytoplankton growth is without question; however, these inorganic forms of nitrogen are not the only utilizable nitrogen forms present in the water column. The ability of phytoplankton to utilize and grow on specific dissolved organic nitrogen (DON) compounds (mostly amino acids and urea) has long been known. Uncertainty in the chemical nature of natural DON has limited studies on its use by phytoplankton as a nitrogen resource. Commonly, DON can be an important component of the total dissolved nitrogen (TDN) pool in estuaries, but the chemical fractions of DON that phytoplankton are known to utilize are generally small. These small component pools of DON however can cycle rapidly and may contribute disproportionately to phytoplankton nitrogen utilization. Given the potential importance of DON as a nutritional source for specific phytoplankton, it is clear that long-term dynamics of DON should be considered when addressing questions of eutrophication. Our research presents an analysis of an 11-year data set of one component of the DON pool, urea. Although encompassing many years, this data set is insufficient to yield insight into inter-decadal trends. The goal of this paper is to first examine the mean temporal and spatial variability of urea concentrations, and the rates of urea uptake and regeneration in Chesapeake Bay, and second, to provide evidence of the importance of external loading of urea and the internal biological cycling of urea in driving these mean temporal and spatial patterns in urea concentration. Figure 1 highlights the increasing importance of urea as a nitrogen substrate as concentrations have increased within Chesapeake Bay.