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Daphnia population growth but not moulting is a substantial phosphorus drain for phytoplankton
Author(s) -
Sommer F.,
Santer B.,
Jamieson C.,
Hansen T.,
Sommer U.
Publication year - 2003
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1046/j.1365-2427.2003.00971.x
Subject(s) - seston , zooplankton , mesocosm , biomanipulation , phytoplankton , daphnia , population , daphnia galeata , biology , bosmina , zoology , phosphorus , eutrophication , environmental chemistry , nutrient , ecology , cladocera , branchiopoda , chemistry , demography , organic chemistry , sociology
SUMMARY 1. Negative effects of zooplankton on the availability of phosphorus (P) for phytoplankton as a result of the retention of nutrients in zooplankton biomass and the sedimentation of exoskeletal remains after moulting, have been recently proposed. 2. In a mesocosm study, the relative importance of these mechanisms was tested for the freshwater cladoceran Daphnia hyalina × galeata . A total of 13 mesocosm bags was suspended in a mesotrophic German lake during summer 2000 and fertilised with inorganic P in order to obtain a total nitrogen to total P ratio closer to the Redfield ratio. D. hyalina × galeata was then added at a logarithmically scaled density gradient of up to 40 ind. L −1 . Zooplankton densities, dissolved inorganic, particulate organic (seston <100 μm), as well as total nutrient concentrations were monitored. Additionally, nutrient concentrations of sediment water removed from the bottom of the mesocosm bags via a manual pump were determined. 3. Seston carbon (C), seston P and total P were significantly negatively correlated with Daphnia densities. The amount of particulate P (∼5–6 μg P L −1 ) sequestered from the seston compartment by Daphnia corresponded roughly to the increase of zooplankton biomass (population growth). Soluble reactive phosphorous (SRP) was at all times high (∼25–35 μg P L −1 ) and possibly unavailable to phytoplankton as a result of P adsorption to calcite during a calcite precipitation event (whiting). P concentrations determined in sediment water were generally <60 μg P m −2 and thus never exceeded 1% of the total amount of P bound in particulate matter of the overlying water column. 4. Seston C : P ratios followed a polynomial second‐order function: At Daphnia densities <40 ind. L −1 a positive linear relationship was evident, which is explained by the stronger reduction of P compared with C in seston, and transfer of seston P to zooplankton. Highest seston C : P ratios of ∼300 : 1 were observed at Daphnia densities of ∼30–50 ind. L −1 , which is in agreement with proposed threshold values limiting Daphnia reproductive growth. At Daphnia densities >40–50 ind. L −1 C : P ratios were decreased because of the strong reduction of seston C at close to constantly low seston P‐values of ∼3–4 μg P L −1 . 5. At least for Daphnia , it may be concluded that – unlike population growth – the sedimentation of faecal pellets and carapaces after moulting seem negligible processes in pelagic phosphorus dynamics.