Impact of the removal of gizzard shad (Dorosoma cepedianum) on nutrient cycles in Lake Apopka, Florida

Abstract

1. The St. Johns River Water Management District removed over 5.4 million kg of gizzard shad (Dorosoma cepedianum) from Lake Apopka, FL during 1993-2005, as a means of reducing lake phosphorus and phytoplankton concentrations and improving water clarity. Other steps included reduction of external nutrient inputs and operation of a treatment wetland. We measured nutrient excretion by Lake Apopka gizzard shad to quantify the nutrient effect of this biomanipulation. 2. Both N and P excretion were significantly affected by fish body mass and temperature. Larger fish had lower mass-specific rates of excretion than smaller fish. 3. High water temperature increased P excretion to a much greater extent than N, resulting in a low N : P of nutrient excretion in midsummer. The N : P of excretion was lower than has been observed in other systems, probably because of higher water temperature. 4. Removal of gizzard shad >200 g prevented the annual release of 45 800 kg N year )1 (3.46 kg N ha )1 year )1 ) and 7700 kg P year )1 (0.62 kg P ha )1 year )1 ) on average. The actual impact on the P cycle varied substantially from year to year (range 7900-78 800 kg N year )1 ; 1200-14 800 kg P year )1 ), primarily because of fluctuations in the catch. 5. On an annual basis, the P directly removed in fish tissues was similar to that removed by the treatment wetland. The P excretion prevented by the removal of fish was approximately 20% of the reduction in external P loading achieved during 1993-2005. 6. In the short term, most of the P demand of planktonic primary producers is met through recycling of P, which greatly exceeds external P loading. Depending on population biomass, phosphorus excretion by the resident gizzard shad population was similar in magnitude to the P release by diffusive flux from the sediments.

Effect of a size-selective biomanipulation on nutrient release by gizzard shad in Florida (USA) lakes

“…The annual removals reported here ranged from 19.1-121.6 kg·ha −1 ·yr −1 (Table V), and the annual removals from Lake Denham (104 ha, 1.4 m mean depth) were 380 kg·ha −1 ·yr −1 during 1990, 88 kg·ha −1 ·yr −1 during 1991, and 30 kg·ha −1 ·yr −1 during 1992 (Godwin et al, 2011). In Lake Apopka, a multipronged management effort has sought to cut external P loading, remove gizzard shad, and remove particulate P via wetland filtration (Coveney et al, 2002Schaus et al, 2010). The combination of these approaches has reduced total P by 54%, reduced Chl a by 37%, and increased Secchi depth by 47% .…”

“…We directly measured nutrient release by 168 gizzard shad collected from Lakes Apopka (n = 58), Dora (n = 46), Eustis (n = 38) and Griffin (n = 26), using methods similar to those of other studies (Brabrand et al, 1990;Gido, 2002) and following the procedures detailed by Schaus et al (2010). Gizzard shad were collected, placed in containers of pre-filtered lake water, and allowed to excrete for 30 min; the difference in nutrient concentrations (before vs. after fish addition) was used to measure excretion.…”

“…Gizzard shad were collected, placed in containers of pre-filtered lake water, and allowed to excrete for 30 min; the difference in nutrient concentrations (before vs. after fish addition) was used to measure excretion. We sampled gizzard shad from all four lakes during Jan. Schaus et al (2010), but are also reported here because of the intentional comparison to another lake. Fish used for excretion measurements spanned the size range available in each lake (7.7−860.0 g wet mass).…”

“…Instead of extrapolating this biomass reduction over an entire year , we focused on the time period immediately after harvest, to minimize any potential seasonal shifts in population biomass, growth, and/or size structure and conservatively compare nutrient effects of harvest among lakes. We focused on lake-wide estimations of P (and not N) because management of these lakes has focused on P (Fulton and Smith, 2008), because other sources of P have been reported for some of these systems (Coveney et al, 2002Schaus et al, 2010) and because we observed that N release differed significantly among lakes. We examined the size-selective nature of the harvest by comparing the size distribution of fish harvested among years and the size distributions of fish harvested by nets of varying mesh size.…”

“…Removals of planktivorous fishes can increase the abundance of larger zooplankton and decrease phytoplankton via trophic cascade mechanisms (Carpenter et al, 1987), but some of the response can be due to changes in nutrient cycling (Vanni and Layne, 1997). The removal of omnivorous and benthivorous fishes can decrease sediment resuspension, directly remove nutrients in fish tissue, and decrease nutrient excretion by fish (Lamarra, 1975;Matsuzaki et al, 2007;Schaus et al, 2010). These processes can interact to lower turbidity and potentially increase macrophyte abundance, which is often linked to the effectiveness of lake restoration (Scheffer, 2001;Søndergaard et al, 2008).…”